Headings named "Map" produce bad HTML

[lavaleri]

When using duvet to parse the following specification, the command succeeds, but produces HTML which will not display in browser (tested Firefox/Chrome):

## Map

something something MUST something something.

This is because the resulting HTML includes something like:

{"id":"map","title":"Map","lines":...}

Specifically id cannot be map. If I change "map" to "map-foo" then the HTML "correctly" displays again.

Ideally, duvet should be able to handle this case. At the very least duvet should error instead of succeeding in this case.