Refactor the `MessageAwaiter` plugin

[huan]

The MessageAwaiter currently added a global Wechaty interface waitForMessage() which polluted the global Wechaty environment.

plugin-contrib/src/contrib/message-awaiter.ts

Lines 21 to 27 in f1d08c7

	declare module 'wechaty' {
	class Wechaty {
	waitForMessage(args: MessageAwaiterArgs): Promise<Message>
	}
	}

Need to remove the above code by refactoring this plugin. Maybe it will be changed to a helper function.

CC: @ssine

Related to PR

• Add MessageAwaiter contrib #12

[huan]

Implemented with wechaty-plugin-contrib@1.0.1