Better mail api


- [x] Class based test suite
- [x] Unify interface for `reply_to`, `from_addr` addresses
- [ ] Unify interface for `to_addrs`, `cc`, `bcc` addresses
- [ ] Fix `bcc` header behaviour, e.g. do not add bcc to message headers but add to recipients list
- [ ] Sanitize address input, e.g. remove `\r`, `\n`, etc.