Associate .torrent files to WebTorrent Desktop (OS X) (#553)

Before this change, .torrent files would only be associated to
WebTorrent Desktop if another torrent client (like Transmission) was
installed on the system.

That's because one of the user's apps needs to define
"UTExportedTypeDeclarations".

On a fresh machine, without Transmission, WebTorrent Desktop now
associates .torrent files correctly.

So it will appear in the "Open With..." menu and the webtorrent
.torrent icon will be used for .torrent files.

Closes #542.

bin/package.js

@@ -182,8 +182,6 @@ function buildDarwin (cb) {
     var infoPlistPath = path.join(contentsPath, 'Info.plist')
     var infoPlist = plist.parse(fs.readFileSync(infoPlistPath, 'utf8'))
 
-    // TODO: Use new `extend-info` and `extra-resource` opts to electron-packager,
-    // available as of v6.
     infoPlist.CFBundleDocumentTypes = [
       {
         CFBundleTypeExtensions: [ 'torrent' ],
@@ -211,6 +209,25 @@ function buildDarwin (cb) {
       }
     ]
 
+    infoPlist.UTExportedTypeDeclarations = [
+      {
+        UTTypeConformsTo: [
+          'public.data',
+          'public.item',
+          'com.bittorrent.torrent'
+        ],
+        UTTypeDescription: 'BitTorrent Document',
+        UTTypeIconFile: path.basename(config.APP_FILE_ICON) + '.icns',
+        UTTypeIdentifier: 'org.bittorrent.torrent',
+        UTTypeReferenceURL: 'http://www.bittorrent.org/beps/bep_0000.html',
+        UTTypeTagSpecification: {
+          'com.apple.ostype': 'TORR',
+          'public.filename-extension': [ 'torrent' ],
+          'public.mime-type': 'application/x-bittorrent'
+        }
+      }
+    ]
+
     fs.writeFileSync(infoPlistPath, plist.build(infoPlist))
 
     // Copy torrent file icon into app bundle