Improvements to video exporting

[stuaxo]

These are a couple of improvements to video exporting, that we may not have time to do before 1.4

Don't use use shell=True

• This is a bit of a pain, it's basically passing the command as a list. It makes it much easier later on in Windows, where the shell may never get good at parsing the command line.

• Move code out of external script and into sbot itself, when called -o and an .mp4 extension.
  This would make things feel much more integrated, but not sure how easy on the current architecture.
  (It may be possible to cheat and call sbot itself in a subprocess).