add required_mods keyword to maps

[sanslash332]

Will be interesting if it possible add a "required_mods" keyword to custom maps, and use that information to filter the maps on select map screen. to aboid the select a incompatible map when you are testing other mods.

example:

title 5012 5014 5023 3004 objective 145 88 required_mods modern, super_techs

Or something similar, if you want to add more granular control, like accept any mod that includes definition for a custom resource_storage.