Add option for numbered grid squares at any zoom level

[hairlesshobo]

Would be nice to have a checkbox to be able to show or hide numbered grids irrespective of zoom level too.

Originally posted by @M0LTE in #470 (comment)

As stated by @M0LTE, please create a checkbox somewhere that allows the numbered grid squares to be shown on the map even a distant zoom levels. Right now, in order to see the numbered squares, you have to zoom in quite a bit and it would be great if, for example, you could zoom out enough to see the entire CONUS and also see the individual numbered grid squares.

Thanks!