Use a transparent background if available

This allows the background of the terminal where ``cmus-lyrics`` is running to be transparent, if the terminal supports it. I always use a transparent terminal, and having the right side of the screen opaque was annoying me a little :smiley:

Since the user may be using a terminal that doesn't support transparency or doesn't have a dark background (xterm screenshot attached), should I add this as a command line switch instead?

![screen](https://f.cloud.github.com/assets/44905/377106/aa53bbaa-a47c-11e2-85a9-cb3a2a9e97d5.png)

![xterm](https://f.cloud.github.com/assets/44905/377118/ccf71b9c-a47d-11e2-9627-02454ddc4e72.png)