Add MessageRegistry

The MessageRegistry is similar to the CommandRegistry in that it maps
string message types to callback functions; however, messages are
dispatched to Atom from outside the application using URIs, and so
should be more restrictive about what they allow.

Signed-off-by: Katrina Uychaco <kuychaco@github.com>

src/application-delegate.coffee

@@ -235,6 +235,14 @@ class ApplicationDelegate
     new Disposable ->
       ipcRenderer.removeListener('context-command', outerCallback)
 
+  onUrlMessage: (callback) ->
+    outerCallback = (event, args...) ->
+      callback(args...)
+
+    ipcRenderer.on('url-message', outerCallback)
+    new Disposable ->
+      ipcRenderer.removeListener('url-message', outerCallback)
+
   didCancelWindowUnload: ->
     ipcRenderer.send('did-cancel-window-unload')