Adding the namespace "plugin" to the rule

[jonrohan]

In the upcoming stylelint 7.0, they're going to be requiring namespaced plugins. This plugin is currently throwing a warning:

Deprecation Warning: Plugin rules that aren't namespaced have been deprecated, and in 7.0 they will be disallowed. See: http://stylelint.io/developer-guide/plugins/

I added the namespace plugin because I thought that would be the easiest, if you want to use another namespace I'd be happy to update the PR.

Thanks! 👋

@timothyneiljohnson