Added a new Lint check for invalid map cordons

This will protect both this repository and http://resource.openra.net from invalid submissions that crash the game. It is also documented at https://github.com/OpenRA/OpenRA/wiki/Mapping#file-format but no one reads manuals.
This doesn't correctly check the right or bottom edges. The following will pass the test, but crash ingame:
```
MapSize: 128,128
Bounds: 120,120,8,8
```