⚡ Add API: SetASDefaultProtocolHandler

This PR adds an API enabling Electron to set itself as the default
protocol handler for a custom porotocl on both oS X and Windows.

For details, please see `docs/app.md`.

Closes #4857

atom/browser/browser.h

@@ -76,6 +76,9 @@ class Browser : public WindowListObserver {
   // Set the application user model ID.
   void SetAppUserModelID(const base::string16& name);
 
+  // Set as default handler for a protocol.
+  bool SetAsDefaultProtocolClient(const std::string& protocol);
+
 #if defined(OS_MACOSX)
   // Hide the application.
   void Hide();