Added support for `format` objects as quality.

This change fixes #16 by allowing the user to pass in a downloaded
format object. Currently the system will only check a predefined list
for it.

The reason I need this is to download videos with high frame rate. HFR
videos do not contain sound, so the video and audio must be downloaded
separately and then combined.

I can provide an example with more descriptive usage and add a new section in the readme for more advanced usage.
I'm not sure that the part about 1080p video and using ffmpeg should be placed here. If it *must* be placed, then I think it belongs more after these options in a separate paragraph. what do you think?

Supplying this option will skip both the `quality` and `filter` options.