[feature request] display albumart of the current playing song

Hello cherrymusic developers,

My colleagues and I are using cherrymusic on our private server and we are very satisfied with its functionality. However, we think the interface would be more informative if the album art of the current playing song can be displayed on the screen. Therefore, I made the following modification to the frontend:

- display an album art image on the bottom-left of the screen (beside jplayer), updated whenever the currently playing song is changed
- a user option to turn off the album art image (for those who don't need it)

I think this functionality may be useful to other users of cherrymusic. Would you please consider merging the modifications into the development branch or implement similar features? Thanks.

Che-Huai Lin

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NOTES:
- The user option is on by default.
- I didn't run `deploy.py`, so files in `res/dist` are still old copies.