Twitch-dl

* Use video owner instead of creator when naming files, since creator can be the
person who uploaded the video, not the author.
* Fix issue with cropping where blank frames were generated at the start of
video (166)
* Add ffmpeg info to `twitch-dl env`

* Fix compatibility with python 3.8

* Download VODs to cache dir instead of temp dir (105, 115, 164)
* Add `download --cache-dir` to allow user to override the default cache dir
* Add `cache` command for viewing cache usage and clearing cache
* Chat rendering improvements: align fonts to a common baseline, scale badges
and emotes

* Fix breaking bug in chat renderer

* Embed additional metadata to videos: `date` (publish date) and `show` (game
name)

* When downloading a video with `--start` and/or `--end` offsets, crop the video
to the exact times given, used to be rounded to the nearest VOD boundary
(usually 10s)