[Save Image Sequence] Improve output path options

- Add option for saving into subdirectories every frame range (range
  between bookmarks, trim editor selections).  If set, the bookmark
  title of the range's starting frame is used.
- Allow saving the frames with sequential numbers as the suffix,
  instead of the frame number

Also return a list with the paths of the saved images, that can be
accessed from another macro as avsp.Last, for example for running
a PNG optimizer afterwards.