Clean up diff formatting in .litdiff files #30

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.

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