Merge: Improve docunits

This PR change a little the behavior of docunits.

First, blocks respect more the markdown specification where empty lines are authorized in code blocks

So this make a single code block and not two:

~~~~
# bla bla
#
#    var a = 1
#
#    assert a == 1
~~~~

Pull-Request: #908
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>