Added a couple of features that I like

I think I've basically poked around every rcs wrapper of latexdiff, and rcs-latexdiff is by far the best.

If you have time, take a look at the features I've added and let me know what you think.

Summary of new features:
- ability to automatically run pdflatex on the diff and then open it in the default PDF viewing program
- ability to diff with uncommitted changes (by omitting NEW argument)