Allow extension to be switched off from the CLI

Currently, it's possible to enable an extension from the CLI by ID using the --extension option. However, this option cannot be used to switch off an extension that is enabled in the playbook.

If the value of the option starts with !, Antora should switch off the extension instead.

antora --extension=!lunr-extension antora-playbook.yml

If the extension is not enabled, the option should have no effect.