refactor proxy code to own lib

[lcmgh]

https://github.com/tafia/hyper-proxy seems not to be maintained anymore and the crate does not support hyper v1. I wonder if it makes sense to refactor such a lib out of the reqwest project?

[seanmonstar]

Yea, as I mentioned recently, I hope to make most of reqwest's features be tower middleware, and that'd include the proxy stuff.