Enhanced video features for the WST Play service

I added some functionality to the WST Play streaming service that I think make for a better experience. This is an addon available on the Firefox addons site and the Chrome webstore.

1. Has a "spoiler-free" mode that removes duration information
2. Stores where you left off watching non-live videos
3. Keyboard actions on videos:
   • spacebar to pause/play
     • may need to click on video first
   • left/right arrows to go back/fwd 10 seconds
   • hold shift + L/R to go back/fwd 5 minutes

"Excellent add-on."

"Works really well!"

"I fucking love it!"

"Watching both live and on-demand videos on WST Play is now made so-so much better."