Add ImageMap Interface and Basic Image Map Commands

[riley-harper]

Closes #48.

This has been pretty difficult. This is the third branch I've been working on, so #48 has a lot of commits listed that won't be merged in. I ran into several design mistakes and issues that convinced me to just redesign the system.

Here's what I've landed on:

• the ImageMap interface represents a general filter that can be applied to an image
• tone maps and other filters like bloom can implement ImageMap and take in parameters in their constructors
• the SceneData object stores a list of image maps to apply to the image before writing it to the output file
• main() is responsible for applying these image maps to the Image before writing it
• Image knows nothing about image maps
• If there are no image maps specified in the scene file, then the program defaults to just doing a basic [0;1] clamp, which makes sense to me. A user can get around this with the im_nop command, which adds an image map that doesn't do anything.

There are two new commands: im_nop and im_basic_clamp. In the future, we can add more interesting commands pretty easily:

• create a new class that implements ImageMap
• in the parser, create an instance of the class with the needed parameters, and add it to sdata.image_maps