Support a Comma separated list (single string) for `tags`

[zachleat]

One issue with the conversion from Ghost to Markdown is how tags are handled. In the export, multiple tags on a post are comma-separated, like this -

---
title: Example Article
slug: example-article
tags: one, two
---

via https://bitsofco.de/from-ghost-to-11ty/#tags

[zachleat]

cc @ireade

[ireade]

@zachleat Awesome, thanks!

[zachleat]

Just for completeness, #1225 wants a space delimited option