SASS/SCSS partials (files beginning with _) are now filtered

Hello Renaud,

I thought this might be what you had in mind when you wrote "better support for sass partials" on the todo list? I use a lot of them and had found no obviuos way to prevent sass from trying to compile them a second time although they were already included in other files. I would be glad if this would make it into your master if the code is ok for you :-)

Kind regards,
Roland