When downloading albums, prefix filenames with their index in the album

Currently, when downloading an album, images are included in the archive as though they were all individual files.  Due to imgur's naming scheme, the filenames are effectively random, so whatever ordering they had in the album is lost.

Sometimes this isn't a problem, but often albums are ordered in a particular way intentionally.  It would make more sense to prefix filenames with their index in the album when downloading them as an album, ie., "1 - (imgur filename).png," "2 - (imgur filename).png," and so on.  This preserves the album ordering when sorting these files by filename, while also preserving the imgur filename so the individual files can be looked up later.