Add option to delete existing destination directory before generating style guide

The styleguide `destination` directory is only created once by `kss-node`.

If I:
1. create a KSS comment in my stylesheet
2. generate the styleguide
3. later remove the comment from my stylesheet
4. regenerate the styleguide

.. the styleguide navigation reflects the change but the previously generated styleguide page is left behind. Over time this introduces cruft/confusion.

Currently I'm working around this with `grunt-contrib-clean`.

Thanks.

Start by tracing how kss-node creates the configured destination directory and where generation options are handled. Done means an opt-in option can remove the existing destination before generation, so pages for removed KSS comments are no longer left behind; verify the behavior with the relevant generation tests.

Written by the indexing model from the issue text.