Merge: Nitunit works with groups and markdown files

A lot of work but quite straightforward. Nit can be a really nice language when doing maintenance of old code.

The first part ot the PR updates the nitdoc to test documentation of groups (as requested by #1201)
The second part of the PR (I planned to do 2 PR but the second was more easy to do than expected) makes that nitunit can also process sand-alone markdown files (documentation, wiki pages, etc.).

Example:

~~~
-- foo/
   |-- README.md
   `-- foo.nit
~~~

~~~sh
# to test all entities of a module (classes, methods, etc.)
$ nitunit foo/foo.nit
# to test all entities of a group (the group and all its modules)
$ nitunit foo
# to test a given markdown file
$ nitunit foo/README.md
~~~

Close: #1201

Pull-Request: #1203
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>