Implement notifications for queries and highlights

- Asks for permission when opening the app if it isn't already
  granted
- Highlights with a notification that stays for 2s
- Clicking on the notification brings that window/channel
  to focus
- Uses document.hasFocus() rather than document.hidden, since
  it seems to be more accurate when the tab is in focus
  but the window is not

Imlements issue #128
Merge pull request #138 from yuvipanda/notifications

Implement notifications for queries and highlights