Add MIME type for `.torrent` files

Serve `.torrent` files with the `application/x-bittorrent` MIME type
instead of the default `application/octet-stream`. This helps improve
the user experience in some browsers (e.g.: in Firefox, the user is
offered the option to open the file with a torrent application, if
it has one installed).

Close h5bp/server-configs-nginx#4

mime.types

@@ -85,6 +85,7 @@ types {
   application/x-stuffit                 sit;
   application/x-tcl                     tcl tk;
   application/x-x509-ca-cert            der pem crt;
+  application/x-bittorrent              torrent;
   application/zip                       zip;
 
   application/octet-stream              bin exe dll;