Merge: extends nitmd to handle manpages output format

A lot of things are done here:

* extension of nitmd to handle more output formats
* new man decorator to transform markdown into basic groff suitable for manpages. Close #1506
* remove dubious usage of definition list in our markdown. Close #1594
* remove the use of pandoc and use nitmd to generate our manpages. Close #824

Pull-Request: #1637
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>