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/index.js

@@ -4,6 +4,7 @@ var electron = require('electron')
 var ipc = require('./ipc')
 var menu = require('./menu')
 var windows = require('./windows')
+var shortcuts = require('./shortcuts')
 
 var app = electron.app
 
@@ -15,6 +16,7 @@ app.on('open-url', onOpen)
 app.on('ready', function () {
   electron.Menu.setApplicationMenu(menu.appMenu)
   windows.createMainWindow(menu)
+  shortcuts.init(menu)
 })
 
 app.on('activate', function () {