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/ipc.js

@@ -1,12 +1,12 @@
+module.exports = {
+  init: init
+}
+
 var electron = require('electron')
 var debug = require('debug')('webtorrent-app:ipcMain')
 var ipcMain = electron.ipcMain
 var windows = require('./windows')
 
-module.exports = {
-  init: init
-}
-
 function init () {
   ipcMain.on('addTorrentFromPaste', function (e) {
     addTorrentFromPaste()