Support UNIX sockets #809

It would be nice to be able to connect to HTTP servers via UNIX socket (e.g. docker.sock). It's mostly interesting for healthcheckers and similar utility type usage. API can be similar to Python's request-unixsocket:

HTTP.get("http+unix://%2Fvar%2Frun%2Fdocker.sock/info")

Alternative API can be similar to cURL:

HTTP.unix_socket("/var/run/docker.sock").get("http://literally-anything-as-host/info")