Add a quality option when transcoding

Currently, the quality is hardcoded. It should be possible, when transcoding video, to set a parameter quality that ranges from 0 to 100.

Then, each receiver implementation should be responsible for handling how to treat that quality.

In the specific case of the Chromecast Receiver, which uses H264 with the CRF option, it should transform 100 into 18 and 0 into 28.