Add webhook posting mode for discord. (#204)

Using it implies to configure a Webhook on discord and set the parameter :
- WebhookURL (New parameter, discord-specific)

Discord API does not allow to change the name of the user posting, but webhooks does.
This makes the relay much more elegant, even if we might lose some more advanced features.

Signed-off-by: saury07 <sacha.aury@gmail.com>

bridge/config/config.go

@@ -77,6 +77,7 @@ type Protocol struct {
 	UseSASL                bool   // IRC
 	UseTLS                 bool   // IRC
 	UseFirstName           bool   // telegram
+	WebhookURL	       string // discord
 }
 
 type ChannelOptions struct {