More customization for diffTool

[andrewjmeier]

I'd like to use Imagemagick's compare to diff snapshots. It expects the format compare [fileA] [fileB] -compose src [outputFileName]. With the way it's currently written, diffTool only works for tools that use the format [tool] [fileA] [fileB]. How about the option to add an errorMessageBuilder block that can construct an error message in whatever format is needed?

[jlcvp]

#668 seems to address this

[andrewjmeier]

Not quite as flexible as what I proposed but that would work for me.

[js]

This is an old discussion, but I added a PR for a diff tool builder closure in #840