Provide an asynchronous interface

[AdamWill]

This was proposed by @RheingoldRiver in #284 , but I don't think it really makes sense to tie it to a 1.0 release, I think we can consider it a separate thing. To me it makes more sense to do a synchronous-only 1.0 release, then an async interface might be more of a 1.1 or 2.0 thing.

As @RheingoldRiver noted, we already have an async shadow-clone at https://github.com/Wadu436/aiomwclient (thanks to @Wadu436 for working on that). It would make sense to look at integrating the functionality into mwclient somehow, but without adding complexity for folks who just want to write synchronous code.