Preserve VCS data when cleaning output; fixes #574

There are several reasons one might want to version a site's output directory. One of those reasons is to facilitate deployment via Heroku, as noted in #574. Another reason might be to allow the user to easily revert to a prior version of the site without having to rely on regeneration via Pelican.

This change modifies the `clean_output_dir` function, preserving `.hg`, `.git`, and `.bzr` directories during the cleaning process.