Have rcirc handle bridge bots

Have rcirc handle bridge bots

* doc/misc/rcirc.texi (Dealing with Bridge Bots): Document new feature.
* etc/NEWS: Mention the new feature.
* lisp/net/rcirc.el (rcirc-markup-text-functions): Add new markup function
(rcirc-pseudo-nicks): Add new local variable.
(rcirc-channel-nicks): Use 'rcirc-pseudo-nicks' for nick completion.
(rcirc-bridge-bot-alist): Add new user option.
(rcirc-bridged-nick): Add new face.
(rcirc-markup-bridge-bots): Add new function.