Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

vendor/github.com/d5/tengo/objects/conversion.go

@@ -0,0 +1,249 @@
+package objects
+
+import (
+	"fmt"
+	"strconv"
+	"time"
+)
+
+// ToString will try to convert object o to string value.
+func ToString(o Object) (v string, ok bool) {
+	if o == UndefinedValue {
+		//ok = false
+		return
+	}
+
+	ok = true
+
+	if str, isStr := o.(*String); isStr {
+		v = str.Value
+	} else {
+		v = o.String()
+	}
+
+	return
+}
+
+// ToInt will try to convert object o to int value.
+func ToInt(o Object) (v int, ok bool) {
+	switch o := o.(type) {
+	case *Int:
+		v = int(o.Value)
+		ok = true
+	case *Float:
+		v = int(o.Value)
+		ok = true
+	case *Char:
+		v = int(o.Value)
+		ok = true
+	case *Bool:
+		if o == TrueValue {
+			v = 1
+		}
+		ok = true
+	case *String:
+		c, err := strconv.ParseInt(o.Value, 10, 64)
+		if err == nil {
+			v = int(c)
+			ok = true
+		}
+	}
+
+	//ok = false
+	return
+}
+
+// ToInt64 will try to convert object o to int64 value.
+func ToInt64(o Object) (v int64, ok bool) {
+	switch o := o.(type) {
+	case *Int:
+		v = o.Value
+		ok = true
+	case *Float:
+		v = int64(o.Value)
+		ok = true
+	case *Char:
+		v = int64(o.Value)
+		ok = true
+	case *Bool:
+		if o == TrueValue {
+			v = 1
+		}
+		ok = true
+	case *String:
+		c, err := strconv.ParseInt(o.Value, 10, 64)
+		if err == nil {
+			v = c
+			ok = true
+		}
+	}
+
+	//ok = false
+	return
+}
+
+// ToFloat64 will try to convert object o to float64 value.
+func ToFloat64(o Object) (v float64, ok bool) {
+	switch o := o.(type) {
+	case *Int:
+		v = float64(o.Value)
+		ok = true
+	case *Float:
+		v = o.Value
+		ok = true
+	case *String:
+		c, err := strconv.ParseFloat(o.Value, 64)
+		if err == nil {
+			v = c
+			ok = true
+		}
+	}
+
+	//ok = false
+	return
+}
+
+// ToBool will try to convert object o to bool value.
+func ToBool(o Object) (v bool, ok bool) {
+	ok = true
+	v = !o.IsFalsy()
+
+	return
+}
+
+// ToRune will try to convert object o to rune value.
+func ToRune(o Object) (v rune, ok bool) {
+	switch o := o.(type) {
+	case *Int:
+		v = rune(o.Value)
+		ok = true
+	case *Char:
+		v = rune(o.Value)
+		ok = true
+	}
+
+	//ok = false
+	return
+}
+
+// ToByteSlice will try to convert object o to []byte value.
+func ToByteSlice(o Object) (v []byte, ok bool) {
+	switch o := o.(type) {
+	case *Bytes:
+		v = o.Value
+		ok = true
+	case *String:
+		v = []byte(o.Value)
+		ok = true
+	}
+
+	//ok = false
+	return
+}
+
+// ToTime will try to convert object o to time.Time value.
+func ToTime(o Object) (v time.Time, ok bool) {
+	switch o := o.(type) {
+	case *Time:
+		v = o.Value
+		ok = true
+	case *Int:
+		v = time.Unix(o.Value, 0)
+		ok = true
+	}
+
+	//ok = false
+	return
+}
+
+// objectToInterface attempts to convert an object o to an interface{} value
+func objectToInterface(o Object) (res interface{}) {
+	switch o := o.(type) {
+	case *Int:
+		res = o.Value
+	case *String:
+		res = o.Value
+	case *Float:
+		res = o.Value
+	case *Bool:
+		res = o == TrueValue
+	case *Char:
+		res = o.Value
+	case *Bytes:
+		res = o.Value
+	case *Array:
+		res = make([]interface{}, len(o.Value))
+		for i, val := range o.Value {
+			res.([]interface{})[i] = objectToInterface(val)
+		}
+	case *Map:
+		res = make(map[string]interface{})
+		for key, v := range o.Value {
+			res.(map[string]interface{})[key] = objectToInterface(v)
+		}
+	case Object:
+		return o
+	}
+
+	return
+}
+
+// FromInterface will attempt to convert an interface{} v to a Tengo Object
+func FromInterface(v interface{}) (Object, error) {
+	switch v := v.(type) {
+	case nil:
+		return UndefinedValue, nil
+	case string:
+		return &String{Value: v}, nil
+	case int64:
+		return &Int{Value: v}, nil
+	case int:
+		return &Int{Value: int64(v)}, nil
+	case bool:
+		if v {
+			return TrueValue, nil
+		}
+		return FalseValue, nil
+	case rune:
+		return &Char{Value: v}, nil
+	case byte:
+		return &Char{Value: rune(v)}, nil
+	case float64:
+		return &Float{Value: v}, nil
+	case []byte:
+		return &Bytes{Value: v}, nil
+	case error:
+		return &Error{Value: &String{Value: v.Error()}}, nil
+	case map[string]Object:
+		return &Map{Value: v}, nil
+	case map[string]interface{}:
+		kv := make(map[string]Object)
+		for vk, vv := range v {
+			vo, err := FromInterface(vv)
+			if err != nil {
+				return nil, err
+			}
+			kv[vk] = vo
+		}
+		return &Map{Value: kv}, nil
+	case []Object:
+		return &Array{Value: v}, nil
+	case []interface{}:
+		arr := make([]Object, len(v), len(v))
+		for i, e := range v {
+			vo, err := FromInterface(e)
+			if err != nil {
+				return nil, err
+			}
+
+			arr[i] = vo
+		}
+		return &Array{Value: arr}, nil
+	case time.Time:
+		return &Time{Value: v}, nil
+	case Object:
+		return v, nil
+	}
+
+	return nil, fmt.Errorf("cannot convert to object: %T", v)
+}