Add a man generator for markup (ronn-style)

[anatol]

Parser struct has a useful method WriteManPage() that allows to generate a man page for command line arguments. I found this idea really handy.

It would be great to add support for other types of manpages. ronn (and its fork ronn-ng) are other popular choices to write man pages. It allows to use markup formatting that is easier to maintain. See an example of a ronn-based manpage here.

Please add a possibility to generate flags manpage in markup format as well. So it will be easier to integrate into projects that use ronn.