message: add addPreSendMessageStanzaProcessor to plugin api
diff: @@ -56,6 +56,10 @@ export default class PluginAPI {
  Pipe.get('afterReceiveMessage').addProcessor(processor, position);
  }
  + public addPreSendMessageStanzaProcessor(processor: (message: Message, xmlMsg: Strophe.Builder) => Promise<any>, position?: number) {
  + Pipe.get('preSendMessageStanza').addProcessor(processor, position);
  + }
  +
  public addAvatarProcessor(processor: (contact: Contact, avatar: Avatar) => Promise<[Contact, Avatar]>, position?: number) {
  Pipe.get('avatar').addProcessor(processor, position);
  }
  