Merge: Some nitunit love

Merge: Some nitunit love

Two small changes

* nitunit works on directories. e.g. `nitunit lib/ contrib/`
* static errors are shown with precise location.

~~~
test_doc3.nit:17,9--15: Syntax Error: unexpected identifier 'garbage'. To suppress this message, enclose the block with a fence tagged `nitish` or `raw` (see `man nitdoc`). (invalid-block)
	#      *garbage*
	        ^
~~~

runtime errors are a more complex beast to track. Two solutions:

* pragma to inject line informations in the generated test program. This need to find a syntax for line pragmas and teach the parser to use them
* post-error management that parse error messages and substitute the locations from the the generated program with the original locations in the real source. Doable but more code is needed.

Pull-Request: #2075
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>