Cleanvid

A few minor changes:

* Added `--re-encode-audio` boolean option, see 31
* Added `--threads <int>`, `--threads-input <int>` and `--threads-encoding <int>` options to pass through to ffmpeg's -threads arguments (see 30); `--threads-input` and `--threads-encoding` take priority, but `--threads` will be used for either one of the others that isn't specified
* minor tweak to swears.txt

Pull from `:6.0` rather than `:latest` image for ffmpeg binaries.

Make padding a floating-point value (e.g., 1.5 seconds).

Removed a leftover volume filter that should have been removed when I added the afade stuff for 13.