Integration with Mocha's diffs via err.expected and err.actual

[ELLIOTTCABLE]

So, Mocha provides diffs on test failures, by examining the thrown AssertionError's .expected and .actual properties. Unfortunately, these are rather redundant, with power-assert's output:

It'd be ideal if, in the context of Mocha, power-assert could only dump its graph-style output, and leave the diffing-style output to Mocha (or at least, instruct Mocha not to do redundant diffing-style output!)

[twada]

@ELLIOTTCABLE Thank you for your feature request. Exactly. Now this feature is in my next milestone.