add a disableForPaths config

This config option is to let the users disable the whitespace package for certain paths or projects they need to keep the whites paces in place.

My personal use case for this is there are projects where we don't want to break the git blame by removing trailing white spaces that have been in place for a long time (legacy!). The ideal would have been a "gitBlameFriendly" option, where it would remove them on lines I modify or lines composed only of spaces, but this is would be more complex and not as self contained so I figured this approach was ok and also easy to implement.

Thoughts?

Closes #30.