Add multi-file input mode

[pps83]

This change enables multi-file input mode, similar to what dssim has. Sample use:

I tried to make the change in a way so that the diff is less intrusive. If you find that the multi-file mode is useful for butteraugli and want it refactored differently, let me know, and I'll update the PR.
The issues with the change:

• heatmap input filename somewhat collides with image2. isMultiFileMode tries to avoid any possible issues.
• ProcessFile possibly adds alpha to rgb1, so, image1 and image2 could be rgb24, but image3 could be rgba, which would change rgb1 to rgba.

In any case, it was quite inconvenient to constantly run butteraugli with different filenames that I constantly needed to compare that I find this option to be a must-have :)