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 guess a separate paragraph would be best, it would keep the descriptions short while still giving the developers some key information (that for some high quality video you'll need to merge the streams). I'll work on it and have something better tomorrow.