Save a copy of the file before the save as well
Features/modifications to match my workflow

- Make ST3 only, particularly since I'm using a version of os.utime()
  that is Py3-only
- Remove unwanted features, like "Compare" and "history_on_close"
- Save a copy before and after the save. Anytime I modify a file, I
  will now have a backup of all versions of that file.
- Make operations synchronous, otherwise there can be unwanted races.
  Hopefully it doesn't slow things down too much.