If we use _`whatever`_ here in ejabberd man pages,
it is converted to *`whatever`* in markdown,
and docs.ejabberd.im/Makefile converts to the proper links
Several documentation callbacks (doc/0 and mod_doc/0) are implemented
and `ejabberdctl man` command is added to generate a man page. Note
that the command requires a2x to be installed (which is a part of
asciidoc package).