Async/await API

[frol]

Do you plan to provide async/await-based API?

P.S. The first step would probably be merging Rust 2018 edition support (#287)

[jhpratt]

@housleyjk I'll attempt to port some portions to async/await if you're willing to accept a PR. If not, I'll likely publish an async-ws crate.

[serzhiio]

It would be very nice to have this crate with async/await features.

[frol]

Well, I am still a bit new to the WS ecosystem, so I just want to point out that there is an on-going effort in porting tokio-tungstenite to tokio 0.2 with async/await API.