Reorganize feature flags to separate out different tls implementations

[Fishrock123]

See #53

I think Ideally it would be like to feature flags for the async-h1 client: h1-client and native-tls or rustls.

This should be acceptable because by the time of the next major, it seems likely we may be switching to using async-h1 by default anyways: http-rs/surf#223 (comment)

[Fishrock123]

This depends on a unstable cargo feature: rust-lang/cargo#8832

Edit: Surf would require that but http-client doesn't.