✨ Add cached #capabilities, #capable?(name), etc

Updated methods:
* `#initialize` - save capabilities in `OK` or `PREAUTH` greeting
* `#capability` - always update saved capabilities
* `#starttls` - always clear capabilities after tagged OK response
* `#authenticate` - clear capabilities or update from tagged OK response
* `#login` - clear capabilities or update from tagged OK response

New methods:
* `#capable?(name)` - the primary API for discovering capabilities
* `#auth_capable?(name)` - returns whether a SASL mechanism is supported
* `#auth_mechanisms` - returns the server's supported SASL mechanisms
* `#capabilities` - cached version of `capability`
* `#capabilities_cached?` - whether capabilities are cached
* `#clear_cached_capabilities` - clears the cache

Also, the docs related to capabilities were reorganized and rewritten.

Fixes #31.

Author: nevans