Merge: nitdoc: Escape paths passed to the shell.

While investigating for a possible source of the recent Jenkins crashes when generating documentation, I found that Nitdoc was not escaping paths in its shell commands properly. So, this PR fix that mistake.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #893
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>