Add support for topic subscribing via WebSockets (#17)

[corentincam]

Here is my (successful) attempt at adding support for topic subscribing (blocking and async).

I added separate features as it adds some dependencies and I guess most people will use this lib to publish messages, not receive them.

Also, I added async-std and tokio features as async-tungstenite has them and we need to choose which module to use according to the async provider.

I'm sure this part could be better but my knowledge of async rust is not huge so don't hesitate to tell me if there's a better way.

Another thing: I couldn't add proxy support easily (see this and this), so I decided to make subscribing and socks features incompatible. Although they technically can be activated altogether, I don't want users to think proxy will work when subscribing to topics (even though there's no API for proxy in subscribers).

Those two last points break the checks with all features, I'm not sure how to resolve that.

I also took the liberty of adding a .rustfmt.toml file because my editor kept reordering imports on save, I can remove it if you don't want it.

[yukibtc]

Thanks @corentincam

I've marked few changes to do.

[corentincam]

About the subscribing/proxy conflict, I decided to add a proxy method to the subscriber builder that panics saying that proxies are not supported by subscribers.

That way the checks can work with all features enabled.