Abort/Resume Support

[rickatnight11]

Currently aborting and retrying a script run will complain about missing files. This doesn't really cause issues, but it's not particularly graceful, and it affects the progress counter.

I propose modifying each line of the digest as the file is renamed (perhaps prepending a hash...simple and easy to check for), which the script can look for to skip. It may be worth going a step further and checking that the resultant file exists, as well.