Added Item IDs to each HTML header tag.
Added additional Markdown extensions.

This should mimic StackOverflow/GitHub.
Added clickable links.
Added Item.parent_documents to mirror child.
Using real objects for export/publish tests.
Moved mock classes/data to the package.
Fixed duplicate iteration.
Skipping CSV/TSV in ref search.
Moved skipped extensions to a setting.
Added maxDiff.
Forced a sort order on child objects.
Documents can't be sorted.

I should really be testing this code before I push.
Forcing a sort order on equal levels.

Travis, why have you forsaken me?
Moved tree publishing to core.
Switched all text files to -text.
More attributes changes.
Moved tree exporting to core.
Refactored publishing code.
Only linkifying with trees.
Update tests for coverage.