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_append.go

@@ -0,0 +1,21 @@
+package objects
+
+// append(arr, items...)
+func builtinAppend(args ...Object) (Object, error) {
+	if len(args) < 2 {
+		return nil, ErrWrongNumArguments
+	}
+
+	switch arg := args[0].(type) {
+	case *Array:
+		return &Array{Value: append(arg.Value, args[1:]...)}, nil
+	case *ImmutableArray:
+		return &Array{Value: append(arg.Value, args[1:]...)}, nil
+	default:
+		return nil, ErrInvalidArgumentType{
+			Name:     "first",
+			Expected: "array",
+			Found:    arg.TypeName(),
+		}
+	}
+}