Support HTTP2 upgrade

[0xAlcibiades]

I'm working up a client which uses reqwest on HTTP2/TLS, and the service I'm integrating with also supports websockets. I came across this library in an endeavour to roll a single client which could operate in HTTP or WS mode, and I see the todo around an HTTP2 handshake needing support in the code. Wondering what the thoughts were, and potential openness to a PR.

[jgraef]

I'm not too familiar with HTTP2 and specifically ALPN and their quirks. I'm very open to PRs!