WIP: Obvious Obsidification

[brekk]

Aiming to make the parser more consistent with the behavior that Obsidian outputs:

• - Add short form of internal links: [[My Internal Link]]
• - Add combinator for both kinds of internal links
• - Add support for ordered lists
• - Update block / paragraph parsers to support internal links
• - Add YAML headmatter parsing (not the full spec, just the parts that obsidian supports)