add native-tls & rustls features

When cross-compiling, it can be convenient to avoid linking to openssl.
This commit adds a 'native-tls' feature which is selected by default so as to
retain current behaviour. The 'rustls-*' features allows using rustls instead of
openssl.

Compiling with, for instance:

````
default-features = false, features = ["rustls-native-roots"]
```

no longer pulls openssl.

Author: fengalin