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

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/Philipp15b/go-steam v1.0.1-0.20180818081528-681bd9573329
 	github.com/Rhymen/go-whatsapp v0.0.0-20190208184307-c9a81e957884
 	github.com/bwmarrin/discordgo v0.19.0
+	github.com/d5/tengo v1.9.2
 	github.com/dfordsoft/golib v0.0.0-20180902042739-76ee6ab99bec
 	github.com/fsnotify/fsnotify v1.4.7
 	github.com/go-telegram-bot-api/telegram-bot-api v4.6.5-0.20181225215658-ec221ba9ea45+incompatible