Run on save

I just came across this plugin today and really like what it does.  One thing I wanted to do with it is have it run whenever I save a file.  I've added an option here so a user can configure it to run on file save.  Let me know what you think.  I'd be happy for any suggestions or improvements to how I implemented it.  I haven't tested it yet on ST2, but the APIs I'm using are supported in both ST2 and ST3.