Add update flag so that unchanged files are not compiled.

This PR is to add an option for --update (update: true), when passed as true we use the SASS native `sass --update source:dest` and will only update if the file is recently changes.

This is great for compiling lots of individual stylesheets, or when a project scaffold contains many different folders so a generous globbing pattern is needed.
I will comment in there as well.

To answer you: I found that if a file hadn't changed, but hasn't been compiled yet (in the case of adding Grunt to an existing project) the stylesheet would compile as an empty file. I am not sure why it does that on the SASS backend, but adding the check to see if the file exists prevented that error.