Make local filenames more unique

Add a unique number at the end of filenames of downloaded recordings (preferably based on the filename of the online file) to prevent skipping videos with the same (local) filename.

Scenario:

For live streams, ETH started to offer two versions (projector and blackboard). Since they do not differ in recording name and the vo-scraper generally uses file names to prevent re-downloads of older clips, this means that only one of the two versions will be downloaded. Worse, when using the history file (--history) this will result in the second recording also being added to the history file, preventing future downloads.

Breakage:

This will change the filename of new downloaded videos and subsequently only prevent re-download if the video on disk already has the new filename.

Therefore, this will cause a re-download of all recordings specified by the user if they rely solely on existing filenames to prevent downloading old videos again.

Users making use of the history file via --history will not be affected.