Proposal: Add "Jekyll-Tagories" core team plugin to whitelist

Per the discussion at [Enable support for tags and categories across all documents]( https://github.com/jekyll/jekyll/pull/5857), the core Jekyll team, led by @ashmaroli has shipped the [Jekyll-Tagories](https://rubygems.org/gems/jekyll-tagories) plugin to enable users to access tags and categories from all document types.

In accordance with the plugin suggestion guidelines:
- [x] The plugin is useful for a vast majority of users – all users with custom collections will benefit.
- [x] The plugin is available as a gem - listed above.
- [x] The plugin is covered with an extensive test suite (@ashmaroli can confirm, but check the Gherkin https://github.com/ashmaroli/jekyll-tagories/tree/master/features)
- [x] The plugin is actively maintained - it's very new, but perhaps @parkr and @ashmaroli can comment on the future here.