Add ability to debug matcher

[reagent]

Sometimes when doing a complex match (e.g. JSON body on POST) it's hard to know why the request is failing and I have to dig into the internals to debug what's going on. It would be good to have a debug flag that would make this easier.

[reagent]

The exception raised by deepEqual will have some details about the discrepancy between the two values -- this may be good for debugging or just reporting match failures.