Implement whitelisting from automatic deletion of aged videos

[vodkapmp]

As mentioned in issue #56, removing videos over x amount of days would be nice if it was per channel/creator.

I would prefer a way to whitelist channels from being deleted after x days age, but I can see people wanting custom days per channel/creator.

[Inrixia]

This should be completely working in 04415a2

Note that this contains breaking changes and deletion will not work with videos downloaded prior to updating. When updating the daysToKeepVideos setting will not be deleted but will also no longer affect anything, a daysToKeepVideos setting will appear on all subchannels which you can customise to your liking.

[Inrixia]

@vodkapmp If you get the chance please test this on the beta channel.