bug-reference auto-setup for IRC, implementation for rcirc

bug-reference auto-setup for IRC, implementation for rcirc

* lisp/progmodes/bug-reference.el (bug-reference-setup-from-irc-alist):
New defvar for configuring bug regexp and URL based on IRC channel and
server names.
(bug-reference--maybe-setup-from-irc): New defun doing the setup given
channel and server.
(bug-reference-try-setup-from-rcirc): New defun calling the above for
rcirc buffers.
(bug-reference--run-auto-setup): Enable the auto-setup for rcirc.
* etc/NEWS: Extend entry describing bug-reference auto-setup.