Add stream title when using mpv

This works for me in Fedora 20, but I haven't yet tested if it causes any issues in Windows.

The problem with this is that it overrides some player args in the user's livestreamer config, such as `-cache 2048`. If I remove `-p mpv`, then the user config is loaded and the stream title is still passed to mpv. However, this depends on the user having `player=mpv` in his config.

Thus the way I have it now is safer, although the other way might be preferable for many people, including myself.