services: add transmission

This service makes use of the default webui. I really like combustion
more, but am willing to use that one instead given the few amount of
time I actually spend looking at it

services/default.nix

@@ -8,5 +8,6 @@
     ./media.nix
     ./nginx.nix
     ./pirate.nix
+    ./transmission.nix
   ];
 }