Support hot-reload for Wechaty events listeners

[huan]

The API might add the following new features:

1. addListener

wechaty.on('message', './message-listener')

If the second parameter is a string instead of a function, then we can use func = require('./message-listener') as the listener function, monitor the file, and reload it if the file had been updated.

2. WechatyInstanceOptions

bot = Wechaty.instance({
    listenerDirectory: './listenerDirectory/'    // this is the directory which contains all listeners.
})

Wechaty will scan ./listenerDirectory/ for the files that the same name as the event, then require it and add it as the listener.

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See Also

1. https://github.com/fastest963/require-reload
2. http://blog.chatie.io/developer/2017/03/20/added-hot-reload-for-bots.html
3. https://blogs.msdn.microsoft.com/typescript/2017/06/12/announcing-typescript-2-4-rc/
4. https://www.youtube.com/watch?v=cGqfM6huJCs
5. Getting the stack trace in a structured form
6. Get v8 stack traces as an array of CallSite objects.
7. A node module for enabling a module to determine its caller.
8. [Design Spec] ESNext import()
9. Hot module replacement for node.js
10. Hot reload tool for node.js

CC @Gcaufy

[Gcaufy]

Looks good for me.
There is another way to do it, may be more convenient to the developers.

bot = Wechaty.instance({
    listeners: './listeners'    // this is the directory which contains all listeners.
});
bot.init();

Maybe you can support both.

[huan]

@Gcaufy I just published a new NPM module as a Hot Module Reload tool named hot-import at here: https://github.com/zixia/hot-import

It supports TypeScript, please give it a try and let me know what you think. :)

[huan]

Landed at v0.8.240.

[huan]

Get to know more about the usage of hot-import listener in Wechaty: #923 #935