Implement tags, categories, series for blog posts

[eorstrom]

Add the ability to add tags and categories to blog posts so they can be grouped by category and tags within the Posts page. Also, if there's a series of blog posts (part 1, part 2, etc.) they can also be grouped together by series.

[bjacquemet]

Hi @eorstrom I think that could serve many people. This repo is open for PR if you feel like adding it! Thanks for your help! Cheers