Parse Markdown blockquotes

> Original issue: https://github.com/PostHog/squeak-react/issues/48

Squeak! supports some Markdown, but apparently not the syntax for blockquotes

[example](https://posthog.com/docs/user-guides/organizations-and-projects#questions)

<img width="700" alt="image" src="https://user-images.githubusercontent.com/154479/172842239-f0e8c170-47b5-4cc8-ad91-45702d7fe516.png">