beets: Add the external beets-alternatives plugin

It's not included in upstream beets but are linked in the documentation
under "Other plugins", see:

http://beets.readthedocs.org/en/v1.3.15/plugins/index.html#other-plugins

I found this one particularly useful for syncing files to varios media
players that refuse to read my FLAC files properly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/tools/audio/beets/alternatives-plugin.nix

@@ -0,0 +1,25 @@
+{ stdenv, buildPythonPackage, fetchFromGitHub, pythonPackages }:
+
+buildPythonPackage rec {
+  name = "beets-alternatives-${version}";
+  version = "0.8.2";
+
+  src = fetchFromGitHub {
+    repo = "beets-alternatives";
+    owner = "geigerzaehler";
+    rev = "v${version}";
+    sha256 = "10za6h59pxa13y8i4amqhc6392csml0dl771lssv6b6a98kamsy7";
+  };
+
+  postPatch = ''
+    sed -i -e '/install_requires/,/\]/{/beets/d}' setup.py
+  '';
+
+  propagatedBuildInputs = with pythonPackages; [ futures ];
+
+  meta = {
+    description = "Beets plugin to manage external files";
+    homepage = "https://github.com/geigerzaehler/beets-alternatives";
+    license = stdenv.lib.licenses.mit;
+  };
+}

pkgs/tools/audio/beets/default.nix

@@ -12,6 +12,9 @@
 , enableThumbnails ? true
 , enableWeb        ? true
 
+# External plugins
+, enableAlternatives ? false
+
 , bashInteractive, bashCompletion
 }:
 
@@ -91,7 +94,10 @@ in buildPythonPackage rec {
     ++ optional enableLastfm     pythonPackages.pylast
     ++ optional enableMpd        pythonPackages.mpd
     ++ optional enableThumbnails pythonPackages.pyxdg
-    ++ optional enableWeb        pythonPackages.flask;
+    ++ optional enableWeb        pythonPackages.flask
+    ++ optional enableAlternatives (import ./alternatives-plugin.nix {
+      inherit stdenv buildPythonPackage pythonPackages fetchFromGitHub;
+    });
 
   buildInputs = with pythonPackages; [
     beautifulsoup4