Add bash completion support

Apart from what stated in the commit message, the script will:
- for cmus-remote
 + complete --server with known hosts or file
 + complete --file or -f with file
 + give hints "about" files in the case "cmus-remote \<tab\>"; give hints about options in the case "cmus-remote -\<tab\>"
 + avoid to complete in the cases --passwd, --seek, -k, --volume, -v, --raw, -C (adding support for raw commands would be nice)
- for cmus
 + complete --listen with known hosts or file
 + avoid further completion when --help, --version or --plugins is the last word written