Switch to comrak's anchorizer

[CosmicHorrorDev]

Fixes #204

comrak's anchorizer is a much better fit for a markdown anchorizer than our home-rolled version

Note: (from the docs)

...

To guarantee uniqueness, an anchorizer keeps track of the anchors it has returned. So, for example, to parse several MarkDown files, use a new anchorizer per file.

Resetting the anchorizer is handled by State (which contains the anchorizer) getting reset before each markdown file gets processed