Refer to wechaty/wechaty-puppet-padchat#112

I think it makes sense to solve this problem. Currently when we call Contact.say() or Room.say() or Message.say(), it is a Promise<void> as the return type. We don't have access to the message that we just sent out. Besides, we don't receive the self-sent-out message in the message event. So currently there is no way to know that which message is the one that we just said.

So I would suggest two options to solve this problem:

• Change the return type for three say() methods to Promise<string>, so we can return the message id back to the developer. As I dived into iPad protocol, the message id is available there.
• Emit message event every time we sent out message, in this way, developer could listen to the event, then get the one they just sent out by matching the content.

I personally prefer to have them both :P
That is to say, we get the message id, then emit a event with that message id and the content, then return the message id back.

What do you think? @zixia Let me know your thoughts, then I could submit a prototype PR.