Command line export for Tiled

Hi! I'm a programmer over at Double Fine and we've been using Tiled to lay out levels for one of our Amnesia Fortnight prototypes. In order to integrate Tiled nicely into our build pipeline, we need the ability to transform tmx files to export formats on the command line. This change hacks some of that behavior into main.cpp. It lets us integrated Tiled into our build server.

Thought it might be of interest to you as well!