Allow to add a root certificate for native-tls

[breard-r]

Disabling the whole certificate verification (#38) is sometimes a way you don't want to follow, even for development purposes. A much safer alternative if to trust an additional root certificate. It would be nice to implement this option in attohttpc, which should be easy since native-tls already implements the add_root_certificate method.

Please note that rustls users already have this option in attohttpc through the client_config method since the ClientConfig object has a RootCertStore field.