Major update and rewrite
 * moved `tests/detect-alsa-output-capabilities.sh` to `alsa-capabilities`
 * modified `alsa-capabilities` to make it suitable to be sourced or
   run from the command line
 * added simple and regexp filtering to `alsa-capabilities`
 * removed alsa interface detection logic out of `mpd-configure`. This
   is now done sourcing `alsa-capabilities`.
 * modified `mpd-configure` to not write to a file by default (see
   updated `README`)