Add alternate fullscreen shortcut ⌘+Shift+F (fix #26)

Uses https://npmjs.com/package/electron-localshortcut to workaround a
bug in Electron (https://github.com/atom/electron/issues/1334).

We can remove `electron-localshortcut` once that bug is fixed.

package.json

@@ -16,6 +16,7 @@
     "create-torrent": "^3.22.1",
     "debug": "^2.2.0",
     "drag-drop": "^2.3.1",
+    "electron-localshortcut": "^0.6.0",
     "hyperx": "^2.0.0",
     "main-loop": "^3.2.0",
     "network-address": "^1.1.0",