Implement AsyncRead and AsyncWrite from futures

[geigerzaehler]

It would be great if this crate would be compatible with futures and provide implementation of futures::io::AsyncWrite and futures::io::AsyncRead. This could potentially be feature gated.

I’d be happy to create a PR

[rousan]

Can you send a PR containing two features: "tokio" and "futures" to switch between them?

The user will use as follows:

To use futures crate:

async-pipe = { version = "0.1", features = ["futures"] }

To use tokio crate:

async-pipe = { version = "0.1", features = ["tokio"] }