Add new scrubber to remove unprintable Unicode characters, fixes #65

I wasn't sure exactly how the top-down/bottom-up and STOP/CONTINUE stuff was supposed to work, but I think this should do the job. Let me know if anything needs to be changed.

I believe that top-down and returning STOP will just run this scrubber once, at the top-level node - correct? This seemed more desirable than removing the characters at every node along the way, but I am not terribly familiar with any performance constraints you might have.