Provide documentation to run from command line

Sometimes I want to run this from the command line. It usually happens when I don't commit to the repo constantly and then I want to generate pretty diffs for a bunch of sketch files at once. This could also potentially allow people to use it as a pre-commit hook.

If you think it's useful, I can take a crack at adding some documentation into docs/ and try to get it to read from the preferences files if added.