The "Cargo feature" `default-tls`, which is enabled by default, is
added, with all TLS support relying on it. This allows using reqwest but
disabling the `native-tls` dependency, by disabling this feature.
Closes#225
- Bumps `native-tls` dependency to 0.2 and adapt code accordingly
- Import code used from `tokio-tls` into `connect_async` and adapt dependencies accordinlgy
- Add an option for using `danger_accept_invalid_certs` inside the `Config` struct