Add a bitrate command line argument to control bitrate of output video

[Netruk44]

Support for specifying a bit rate for the output video file on the command line (--bitrate)

Potential fix for #19

[Netruk44]

Note that this is just what worked for my specific use case. It looks like this probably does nothing for 2D video upscaling, but I'm not sure how best to add that.

Feel free to close the PR without merging if this isn't the way you want to go about doing this kind of thing :)

[Netruk44]

Adding a note here that there's also an alternative implementation available on my auto-bitrate branch: https://github.com/Netruk44/fx-upscale/tree/auto-bitrate

This version automatically calculates a bitrate for the 3D video if one hasn't been passed in on the command line.

I can make a PR with that branch instead, if you prefer that implementation.