feat(client): add Proxy support

[seanmonstar]

This works by configuring proxy options on a Client, such as client.set_proxy("http", "127.0.0.1", "8018").

Closes #531

[ishbir]

Seems like this merge was undone by d35992d. Was it on purpose?

[seanmonstar]

@ishbir sort of. The commit was on purpose, but the hyper Client won't be doing as much automatically. In 0.10, it's possible to handle proxying completely outside the hyper crate. You can connect to any host, and then the handler can use request.set_uri(RequestUri::AbsoluteUrl(target_url)) which will send a request to the host and proxy it (assuming the host knows how to proxy).

Likewise, to provide proxying to https targets, a Connect type can tunnel to the target host before returning a ready TcpStream.