Method for enabling PUBLISH_MKDOCS when calling publisher.publish() directly

[reiterative]

The modifications to the Markdown publisher to support rendering by mkdocs are triggered with the setting PUBLISH_MKDOCS.

This works fine when running doorstop publish from the command line, but it would be useful to have a way to provide this setting when calling publisher.publish() directly by another Python script. Perhaps this could be done by extending utilities.configure_settings to include a way to set PUBLISH_MKDOCS, and call that before calling publisher.publish()?