Add tags support

[Charca]

I'm thinking we could add a new tags front-matter variable to filter posts by tag, something like:

---
title: Hello World 👋
date: "2019-06-11T08:38:00.000Z"
tags: design, random
---

And then create a new route, similar to blog/index, but to filter posts by tag:

/blog/tag/[slug].svelte
/blog/tag/[slug].json.js

[JHethDev]

To output the tags as an array from the front matter you should format like this:

---
title: Hello World 👋
date: "2019-06-11T08:38:00.000Z"
tags:
  - design
  - random
---