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_convert.go

@@ -0,0 +1,155 @@
+package objects
+
+func builtinString(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*String); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToString(args[0])
+	if ok {
+		return &String{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinInt(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*Int); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToInt64(args[0])
+	if ok {
+		return &Int{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinFloat(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*Float); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToFloat64(args[0])
+	if ok {
+		return &Float{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinBool(args ...Object) (Object, error) {
+	if len(args) != 1 {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*Bool); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToBool(args[0])
+	if ok {
+		if v {
+			return TrueValue, nil
+		}
+
+		return FalseValue, nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinChar(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*Char); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToRune(args[0])
+	if ok {
+		return &Char{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinBytes(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	// bytes(N) => create a new bytes with given size N
+	if n, ok := args[0].(*Int); ok {
+		return &Bytes{Value: make([]byte, int(n.Value))}, nil
+	}
+
+	v, ok := ToByteSlice(args[0])
+	if ok {
+		return &Bytes{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}
+
+func builtinTime(args ...Object) (Object, error) {
+	argsLen := len(args)
+	if !(argsLen == 1 || argsLen == 2) {
+		return nil, ErrWrongNumArguments
+	}
+
+	if _, ok := args[0].(*Time); ok {
+		return args[0], nil
+	}
+
+	v, ok := ToTime(args[0])
+	if ok {
+		return &Time{Value: v}, nil
+	}
+
+	if argsLen == 2 {
+		return args[1], nil
+	}
+
+	return UndefinedValue, nil
+}