Add custom image filtering, empty directory checks and more

Completing two points of my to-do list with

• added support for filtering with custom images
• added check for empty directories
  Especially the first point felt really necessary to avoid doing a new release for every other new aggregator image...

Additionally, I discovered that duplicate checking (or technically any image comparison) isn't as flawless as I was hoping. Essentially, there may be false positives which is especially problematic for the duplicate check. As a temporary solution, I disabled duplicate checking by default and added a section to the readme explaining the issue in slightly more detail. It also includes a "workaround" on how to use the feature until I'm more confident in its reliability.

Finally, I did some other small fixes and optimizations...

First public release!

Although far from complete or perfect, I believe the current version to be ready for experimental use. While I plan to add and improve functionality, I hope to gather feedback from actual users testing the script to make the code as stable as possible.

archive directory entry threads

Turns out, threading on a directory level doesn't yield any performance improvement, rather the opposite is the case.
I suppose it's a mix of added overhead and reading a directory out of order slowing down I/O that's causing the performance degradation.
Once a directory has been cached and I/O doesn't bottleneck anymore, there are no performance advantages either so I don't think it's worthwhile to keep the feature, at least not in its current state.