New Ustream plugin option "--ustream-video-only"

When trying to get Ustream video only stream (like ISS HD live stream, http://www.ustream.tv/channel/iss-hdev-payload) livestreamer 1.8.0 hangs for 60 seconds and then exits with error "Read time out". This patch add Ustream plugin option "--ustream-video-only" for grabbing such streams.