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.

main/shortcuts.js

@@ -0,0 +1,13 @@
+module.exports = {
+  init: init
+}
+
+var electron = require('electron')
+var localShortcut = require('electron-localshortcut')
+
+function init (menu) {
+  // ⌘+Shift+F is an alternative fullscreen shortcut to the one defined in menu.js.
+  // Electron does not support multiple accelerators for a single menu item, so this
+  // is registered separately here.
+  localShortcut.register('CmdOrCtrl+Shift+F', menu.toggleFullScreen)
+}