Add marker generation feature

[trubb]

I'm sick and tired of having to manually create the markers I place on the map.

To aid me I would like to be able to enter the following:

• a starting position that defines the top left of the marker grid: [1234,5678]
• a list of forbidden letters that are assigned to some unit already: [K, A, B, C]
• optionally a range of numbers: [0,9], [1,15]
• optionally a marker type: triangle, dot etc
• optionally a scale: 0.1-1
• probably something else as well but hey

The markers should be placed in columns by letter and rows by number on the game map, with enough spacing that it's easy to grab them.
The output should be pretty-printed, with line breaks after each marker, to make it easy to share the result.