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.

bridge/config/config.go

@@ -168,8 +168,10 @@ type Gateway struct {
 }
 
 type Tengo struct {
+	InMessage        string
 	Message          string
 	RemoteNickFormat string
+	OutMessage       string
 }
 
 type SameChannelGateway struct {