Create framework for IRCv3 support

Create framework for IRCv3 support

* rcirc.el (rcirc-implemented-capabilities): Add new variable
(rcirc-requested-capabilities): Add new variable
(rcirc-acked-capabilities): Add new variable
(rcirc-connect): Request capabilities from rcirc-implemented-capabilities
(rcirc-process-regexp): Extend rcirc-process-regexp with tag support
(rcirc-tag-regexp): Add new tokenizer for tags
(rcirc-message-tags): Add new variable
(rcirc-get-tag): Add new function
(rcirc-process-server-response-1): Parse message-tags
(rcirc-handler-CAP): Add new handler for capability requests