Add scripting (tengo) support for every outgoing message (#806)

Adds a new key OutMessage under [tengo] table, which specifies the location of the script that
will be invoked on each message being sent to a bridge and can be used to modify the Username
and the Text of that message.

The script will have the following global variables:
read-only:
inAccount, inProtocol, inChannel, inGateway
outAccount, outProtocol, outChannel, outGateway

read-write:
msgText, msgUsername

The script is reloaded on every message, so you can modify the script on the fly.

The default script in https://github.com/42wim/matterbridge/tree/master/internal/tengo/outmessage.tengo
is compiled in and will be executed if no script is specified.

contrib/outmessage-irccolors.tengo

@@ -0,0 +1,7 @@
+// See https://github.com/42wim/matterbridge/issues/798
+
+// if we're not sending to an irc bridge we strip the IRC colors
+if outProtocol != "irc" {
+    re := text.re_compile(`\x03(?:\d{1,2}(?:,\d{1,2})?)?|[[:cntrl:]]`)
+    msgText=re.replace(msgText,"")
+}