Clean up diff formatting in .litdiff files

[paigeruten]

Right now diffs in .litdiff files are just the output of git diff, which really isn't necessary. They could be made a little more human-readable.

• [x] Remove unnecessary diff --git ... header, come up with more elegant way of starting a diff.
• [x] Show step number somewhere
• [x] Consider adding line numbers to every line in the diff (and then maybe simplify the hunk headers?)
• [ ] Preserve context line to the right of @@ in hunk headers (#18)