Adds an ability to hide thumbnails of videos you've already watched.
Adds a "Hide/Show watched" button which allows you to hide thumbnails from videos you've already watched. I wrote it so that I didn't miss any videos on the Subscriptions page, but this also works on the main page and on the sidebar while watching videos. This doesn't apply to playlist pages, and that's by design.
As a stopgap measure, greying out watched thumbnails can now be disabled by opening developer console (Ctrl + Shift + K on Firefox, Ctrl + Shift + J on Chromium and derivatives), typing localStorage.setItem("dontGreyOutWatchedThumbnails", true) then pressing Enter, and reloading the page. I might make a GUI for it in the future.
--- CHANGELOG ---
1.3: Added a way to disable greying out watched thumbnails.
1.2: Fixed videos being incorrectly marked as watched. Also, watched videos are reverted to being clickable (this was a bit confusing), but still greyed out.
1.1: Watched videos are now non-clickable and greyed out.
