Support seeking by thumbnail in the HTML5 player

[cooperdk]

Feature:
Add support for seeking by clicking a certain thumbnail in a preview set.
FX, have 12 thumbnails tagged or named by location in the video's duration, and clicking that will jump to that location in the video. (video.mp4#t=00:20:30 will move to minute 20, second 30 of the video)

Importance:
Nice to have.

[cooperdk]

Found this to generate the thumbnail sprite:
https://github.com/flavioribeiro/video-thumbnail-generator
Will need to create a vtt generator to support this, or use one that's already on Github (found elsewhere).