Support for tiled maps

[zacharycarter]

Support for tiled maps would be nice.

Should support loading http://www.mapeditor.org/ maps.

[AjBreidenbach]

Hey, I think I'd maybe like to work on that.
Just to be clear, it's this format, right? http://doc.mapeditor.org/reference/tmx-map-format/

Would you like it to be implemented through the load proc that returns a TiledMap in tiled_map.nim?

Also might you want to refactor the Asset type from a case object to using inheritance? And maybe we could move tile related types into tiled_map.nim.