Regression: Images are not output if comparison passes, even if requested

Introduced in https://github.com/myint/perceptualdiff/pull/16

This Pull request effectively introduced a regression in perceptual diff, since this was a feature of version 1.1:

1.1 - Added colorfactor and downsample options. Also always output difference file if requested. Always print out differing pixels even if the test passes.
