Add Batch Support to GUI

[jrg94]

With the way things are shaping up in v2.2.0, I'm thinking we could pretty easily implement batch support in the GUI. I'm thinking it would be cool to add an old school image scroll feature which would let you "see" all of the images that you're rendering but not all at once. I'm imagining that old iTunes screen which let you flip through album covers in a line. Instead, we'll just let you preview one at a time. Then, give you arrows at the bottom to cycle through all the previews.

[jrg94]

Also, it's worth noting that v2.2.0 will likely support this by accident. Users will be able to specify -b on the command line of the gui which will launch the GUI in batch mode. Of course, there is no way for the user to actually take advantage of this because -p and -b are linked. If the user loads the path, they'll load a folder which will break process_images.

I'll have to think about how to handle this before we push #67.