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/builtin_json.go

@@ -0,0 +1,54 @@
+package objects
+
+import (
+	"encoding/json"
+)
+
+// to_json(v object) => bytes
+func builtinToJSON(args ...Object) (Object, error) {
+	if len(args) != 1 {
+		return nil, ErrWrongNumArguments
+	}
+
+	res, err := json.Marshal(objectToInterface(args[0]))
+	if err != nil {
+		return &Error{Value: &String{Value: err.Error()}}, nil
+	}
+
+	return &Bytes{Value: res}, nil
+}
+
+// from_json(data string/bytes) => object
+func builtinFromJSON(args ...Object) (Object, error) {
+	if len(args) != 1 {
+		return nil, ErrWrongNumArguments
+	}
+
+	var target interface{}
+
+	switch o := args[0].(type) {
+	case *Bytes:
+		err := json.Unmarshal(o.Value, &target)
+		if err != nil {
+			return &Error{Value: &String{Value: err.Error()}}, nil
+		}
+	case *String:
+		err := json.Unmarshal([]byte(o.Value), &target)
+		if err != nil {
+			return &Error{Value: &String{Value: err.Error()}}, nil
+		}
+	default:
+		return nil, ErrInvalidArgumentType{
+			Name:     "first",
+			Expected: "bytes/string",
+			Found:    args[0].TypeName(),
+		}
+	}
+
+	res, err := FromInterface(target)
+	if err != nil {
+		return nil, err
+	}
+
+	return res, nil
+}