Remove virtual environments

This PR adds the ability to remove virtual environments using the `--environment` command line option.  The virtual environment directory can be specified via the `virtualenv-dir` option either on the command line or in a *setup.cfg*.  If it is not specified, then the `$VIRTUAL_ENV` environment variable will be used.

This is the first feature in the 0.0.3 release series and officially moves this package into "alpha" territory.