Add manpages

Manpages are in markdown format and they're converted into groff man
by pandoc.
Update packaging to build manpages in build time.

* Add manpages for clara.
* Update packaging to build manpages in build time.
* Add b-d on pandoc.
Rename from command to plugin
Make "clara help <plugin>" show the manpage of the plugin and "clara help", the generic manpage of clara.