Option to include collision layer in tmxrasterizer

I recently needed the ability to convert my maps to an image using tmxrasterizer, but all my maps used a layer named "collision" that I wanted to export. It wasn't until I poured through the source of tmxrasterizer that I realized that the layer named "collision" was hardcoded as one to be omitted from the rasterization.

This change is simply just another command-line option to tell it to include the collision layer in the export. 
After changing it for my own use, I thought I'd submit my changes back in case you'd like to include them.  If not, no worries.

Thanks!

Nathan