Decouple compare from test

Hi @garris ,

As far as I have seen there is no way to run compare on its own, it just runs automatically after creating the test screenshots have been finished. Is there an easy way to decouple the two?
Some backwards compatibility issues may arise from this, though. I have some ideas to fix that:
- Add an option to test --noCompare. When this is set, the test will skip the compare part
- Add a new command like testOnly (disclaimer: I'm not good at naming things)

And the compare command should have a --timestamp or something like that. This would be useful when you have 5 tests already, but you want to view older ones in the report.

What do you think?