Adds file export, auto-save and watch files.

- Adds file export, allowing frames to be exported as invididual
  PNGs rather than a zip archive.
- Adds auto-saving, removing the need to manually click a
  bunch of buttons each export.
- Adds watch files for hooking into a pipeline.
- Refactors code and adds comments.
- Updates README with auto-save and watch files examples.