gajim: add extraPythonPackages option

For exampole, this allows to add python-axolotl to PYTHONPATH,
which is needed for OMEMO OTR messaging plugin.

pkgs/applications/networking/instant-messengers/gajim/default.nix

···
 , enableRST ? true
 , enableSpelling ? true, gtkspell ? null
 , enableNotifications ? false
+, extraPythonPackages ? pkgs: []
 }:
 
 assert enableJingle -> farstream != null && gst_plugins_bad != null
···
   ] ++ optionals enableJingle [ farstream gst_plugins_bad libnice ]
     ++ optional enableE2E pythonPackages.pycrypto
     ++ optional enableRST pythonPackages.docutils
-    ++ optional enableNotifications pythonPackages.notify;
+    ++ optional enableNotifications pythonPackages.notify
+    ++ extraPythonPackages pythonPackages;
 
   postInstall = ''
     install -m 644 -t "$out/share/gajim/icons/hicolor" \