Support for --debug option

[ukch]

From mocha --help

...
Options:
    ...
    -d, --debug                             enable node's debugger, synonym for node --debug
    ...

It would be helpful if mocha-webpack could add an equivalent option to allow debugging of test code.

[astorije]

@ukch, do you think you could open a PR for this maybe?

[haltman-at]

Note that mocha has both --debug and debug which are a little different; ideally mocha-webpack should support both.