Excluding streams of a protocol that's not in the priority list

### Problem

This list all protocols

	python -c "import livestreamer_cli.main; livestreamer_cli.main.main()" justin.tv/tsm_theoddone --stream-priority=rtmp
	Found streams: 240p, 360p, 480p, 720p, 720p+ (best), mobile_high, mobile_low (worst)

### Solution

This list all `rtmp`

	python -c "import livestreamer_cli.main; livestreamer_cli.main.main()" justin.tv/tsm_theoddone --stream-priority=rtmp
	Found streams: 240p (worst), 360p, 480p, 720p, 720p+ (best)

This list all `rtmp` and `hls`

	python -c "import livestreamer_cli.main; livestreamer_cli.main.main()" justin.tv/tsm_theoddone --stream-priority=rtmp,hls
	Found streams: 240p, 360p, 480p, 720p, 720p+ (best), mobile_high, mobile_low (worst)