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/script/conversion.go

@@ -0,0 +1,33 @@
+package script
+
+import (
+	"github.com/d5/tengo/objects"
+)
+
+func objectToInterface(o objects.Object) interface{} {
+	switch val := o.(type) {
+	case *objects.Array:
+		return val.Value
+	case *objects.Map:
+		return val.Value
+	case *objects.Int:
+		return val.Value
+	case *objects.Float:
+		return val.Value
+	case *objects.Bool:
+		if val == objects.TrueValue {
+			return true
+		}
+		return false
+	case *objects.Char:
+		return val.Value
+	case *objects.String:
+		return val.Value
+	case *objects.Bytes:
+		return val.Value
+	case *objects.Undefined:
+		return nil
+	}
+
+	return o
+}