Additional block types

[mcginty]

This is a list that will need to be updated as Notion fills out their API support.

• Columns (they behave similarly to blocks with children, so this is mostly a styling challenge)
• Embeds (may or may not actually want these)
• External images
• Toggles
• File/PDF
• Equation (KaTeX support needed)
• Bookmarks
• Table of Contents (maybe)
• Links to other blog posts by their internal Notion links

[zohaad]

My pull request #5 adds equation support.