Hide specified message types sent by lurkers

Hide specified message types sent by lurkers

* erc.el (erc-display-message): Abstract message hiding decision
to new function erc-hide-current-message-p.
(erc-lurker): New customization group.
(erc-lurker-state, erc-lurker-trim-nicks, erc-lurker-ignore-chars)
(erc-lurker-hide-list, erc-lurker-cleanup-interval)
(erc-lurker-threshold-time): New variables.
(erc-lurker-maybe-trim, erc-lurker-initialize, erc-lurker-cleanup)
(erc-hide-current-message-p, erc-canonicalize-server-name)
(erc-lurker-update-status, erc-lurker-p): New functions.  Together
they maintain state about which users have spoken in the last
erc-lurker-threshold-time, with all other users being considered
lurkers whose messages of types in erc-lurker-hide-list will not
be displayed by erc-display-message.