🗑️ Add deprecation warnings to .new and #starttls

[nevans]

Preparing for a (backwards-incompatible) secure-by-default configuration, Net::IMAP.default_ssl will be used when no explicit port or TLS setting is provided.

TODO: should truthy default_ssl be used to config params when port is 993 but ssl is implicit? Another var?

Moved all deprecated option handling to DeprecatedClientOptions, which is prepended to Net::IMAP.

Additionally, split initialize up into small helper methods making it easier to understand at a glance.

This is a first step towards fixing #118.