Add tuple() to exporter.

Hi there.

I modified the script to get rid of the error about endswith() expecting str or tuple, not frozen set. Changing this if condition in line 189:
if img_tmp_path.lower().endswith(bpy.path.extensions_image):

To:
if img_tmp_path.lower().endswith(tuple(bpy.path.extensions_image)):

Solves the problem perfectly. It's too little of a change to clone and make a pull request, so I will leave this little fix as an issue here. I exported a couple of times with no problems, I can't tell for sure if it will ever present some undesired behavior, but so far it does the job.

Thanks.