Smarter trim_trailing_white_space_on_save

TTWSOS (trim_trailing_white_space_on_save) is awesome most of the time, but often makes me look like an ass in version control.  Committing a one-line change often turns into a massive commit with tons of code insertions and deletions.

It would be great to have more control over this feature...  

For example, I don't want to TTWSOS "dirty" files that have existing trailing whitespace.  I just want to maintain TTWSOSification in already "clean" TTWSOSified files.

Of course it would be nice to be able to force a TTWSOSification on a dirty file.

This would keep commits nice and clean and would resolve my love/hate relationship with this feature!