Add whitelisted channel support

[mat926]

Let me add channels where the addon wont modify the video thumbnail.

[pietervanheijningen]

This has been something i've attempted, but the issue comes down to it being difficult to detect what thumbnail belongs to what youtube channel. As all the extension see's is just the url of the thumbnail, and based on that it then needs to decide in a couple of milliseconds what channel it belongs to.

I have some code up on the test_subs_support branch that makes a call to the youtube api for every thumbnail, but it was just way too slow and resulted in a horrible experience.

If anyone knows a way to do it i'd love to implement it though!