Temporarily store content in localStorage #205

Contents of dirty files are cached to `localStorage` on `pagehide` event (fired on page refresh, close, or navigating away) and restored (or removed from `localStorage` if expired) on initEditor. Undo command in the editor will undo the stash apply, reverting the file to its loaded state from GitHub, but won't reset the `dirty` flag. `app.model.content` is updated with the current content when the stash is applied. Feel like this needs some UI notification to make the user aware that content has been restored and possibly give an explicit option to revert to the content loaded from remote?