Enhancements to diffTool?

[JackYoustra]

My current workflow for image snapshot tests is usually imagemagik's compare tool, so I'll have a command like

compare "1.png" "2.png" png:- | open -a Preview.app -f

Unfortunately, the framework only lets me specify the "compare" step instead of providing the rest of the command. I was thinking on changing / adding a closure version of diffTool that'd be able to do this. Is there any interest in this or any ideas on how people would like a more generalized diffTool?

[tisohjung]

I would appreciate it if anyone can review this PR

#668

[endanke]

@JackYoustra you can probably make an alias to your command which would take the two parameters and run the whole thing.