Map List Checkboxes for sendou.ink/maps

Adds checkboxes to the map list generated by sendou.ink/maps.

Installation

Make sure you have user scripts enabled in your browser (these instructions refer to the latest versions of the browser):
- Firefox - install Tampermonkey or Greasemonkey.
- Chrome - install Tampermonkey.
- Opera - install Tampermonkey or Violent Monkey.
- Safari - install Tampermonkey.
- Dolphin - install Tampermonkey.
- UC Browser - install Tampermonkey.
Install:
- Install a script directly from GitHub by clicking on the "install" link below.