Thumbnails!

Depending on how many videos are in the folder, it can take anywhere from 1 second (< 20 videos) to ~1 minute (100s of videos). I decided it was better to always display the directory listing immediately, rather than wait for all the thumbnails to generate first.

This branch auto-generates thumbnails for all potentially valid videos in the current directory as separate async ffmpeg threads, in parallel. This means, the first time you view a directory you won't see thumbnails yet, but it'll still display the directory listing immediately, while generating available thumbnails in the background. Thumbnails for each visited directory are saved to a hidden '.thumbs' subdirectory within it, and served by a new '/thumb' static media route.