Add formatting commands to rcirc

Add formatting commands to rcirc

* etc/NEWS: Mention new command
* lisp/net/rcirc.el (rcirc-format): Add new utility function.
(rcirc-unformat):  Add new command.
(rcirc-format-bold): Add new command.
(rcirc-format-italic): Add new command.
(rcirc-format-underline): Add new command.
(rcirc-format-strike-trough): Add new command.
(rcirc-format-fixed-width): Add new command.
(rcirc-mode-map): Bind formatting commands.
(rcirc-multiline-minor-mode-map): Bind formatting commands.