feature: Pass client configuration to `mocha.setup method.

Now all configuration in `client.config.mocha` is passed to the
setup method of mocha. The default ui style of `'bdd'` is still
set if nothing else is supplied. Also the reporter will be always
overridden as this needs to be our custom one.

Fixes #13.

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I know that tests are missing but I'm not sure how it's possible to test this with the current testing setup.