Added "deadbeef" package

This is a player that I use daily. First pull request to NixOS, so please point out my mistakes!

I'm not really sure that I'll be able to maintain this package in a long term -- I'm just evaluating NixOS for the first time coming from Arch Linux. I'm willing to do the job if I move, through. I've listed myself as a maintainer for now -- should I have done this?