Improve handling command line arguments

[morgant]

This was a quick hack, on my part, so currently the one -s option has very rudimentary handling and there's zero input validation. Ideally, there would be validation requiring:

• Require one or more arguments
• Validate that the resulting file doesn't already exist (ffmpeg will prompt to overwrite, but since we're using two processes with pipes, I'm not sure if that works)
• Easily support future options (beyond -s), especially:
  • -h for help
  • -q for quiet
  • -v for verbose
  • Maybe specifying FPS or container file type?
• Display help if invalid options or number of arguments are provided