Mark tags up with the `p-category` microformat markup

As part of #407 (closed), we want to add p-category tags to our taxonomy terms within posts.

However, from discussions in the IndieWeb chat0, there is no real distinction between categories/tags in the spec, so the community generally use tags instead. I'll follow suit for my own posts.

Closes #407 (closed).

Additionally, add a test to check that we've got our p-category markup set.