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, does `app` hold the unmodified content anywhere for comparison purposes?

cc @dhcole