Ability to remove original failure message when user message is provided.

[sunshinejr]

It would be awesome to be able remove the original failure message from the failure string, meaning:

Hey these aren't equal!
Expected to equal <2>, got <1>

could become:

Hey these aren't equal!

If I'm providing my custom message I rarely need an original failure, especially when I generate reports that I want to pass to other people - the second line is usually just a noise.

Would this be something that could be introduced in Nimble? I started looking at the possible implementation, and this place seems like a good place to start, but then I wonder how we would be able to specify if we want the original failures in the messages or not.

Ideally we could just opt-in using a single property, something like:

Nimble.showOriginalFailuresWithUserDescriptions = false