Refactor latexdiff pipeline

Make the pipeline for latexdiff with git history lets us re-use the
pipeline from other commands, such as watch --diff.
Add --diff to watch command

This will run a latex-diff based compile whenever the document is
changed!

Made default --diff the HEAD, see http://stackoverflow.com/a/17272318
for how to do this.

Update README with documentation on how to run this command.

Fix #6.