Add in diffToolSuffix to support ImageMagick cli

[tisohjung]

Added diffMessageConverter to support other tools like ImageMagick
which has commands that looks like magick compare "${image1}" "${image2}" diffImage.png

    diffMessageConverter = { "magick compare \"\($0)\" \"\($1)\" diff.png ; open diff.png" }

[mlostekk]

any idea when this will be merged?

[chosa91]

What about rework these two into a reusable, tool-independent message converter?
I'm suggesting this due to the following. Although, the solution offered works fine with these 2 tools, but what about using separate switches to prefix each item (for example: another_img_diff_tool -d 42 -i img1.png -i img2.png?

Something like this (at the moment I have not bothered to deprecate the old variables, but that would complete the picture):

public var diffMessageConverter: (String, String) → String? = { "\(diffTool) \($0) \($1)" }

[tisohjung]

good suggestion
8959584

[stephencelis]

Closed by #840.