Establishes conditional compilation for all integration with the
native_tls crate in this crate. Since native_tls has been deeply
integrated into this crate for a long time, we want to maintain
backwards compatibility by making this feature part of the default.
For a consumer of this crate to "opt-out", including this in
cargo.toml:
```
[dependencies.imap]
version = 0.16.0 # Replace this with the correct version
default-features = false
```
See the conversation on Github for details on this approach:
https://github.com/jonhoo/rust-imap/issues/123
The underlying Rust library for crypto `ring` using multiple
`#[cfg_attr()]` properties together which stabilized in rust 1.33.0:
https://github.com/rust-lang/rust/blob/stable/RELEASES.md#language-4
Previously, everything else in this could be compiled with rust 1.32.0,
so this may constitute a breaking change.
