Introduce _create_node helper method
Use _parse_square_bracket_link to parse bracketed links

This is to separate the logic from the rest of BUILD, as well as
to provide a point for overriding/augmenting the logic for extensions
in the future
Flesh out wrapped links test
Parse out bracketed external links
Add more attributes to ExternalLinkURI
Always set link_uri for external links
Test wrapped URIs with a fragment
Set up wrapped external link sections correctly
Add some design comments
Bump required version of Perl in dist.ini
Add definedness check
Add test for internal links
Flesh out test for headers
Create constant for max header level
Add level attribute to Header element
Add _requires_paragraph helper method
Lazily add paragraph elements
Load header elements outside of paragraphs
Flesh out interwiki link test
Implement inter-wiki links