Add changelog entries for ExternalLinks.
Add ExternalLink Node type.
Implement ExternalLinks as tokens and in the builder.
Add external link contexts; reorder stuff for consistency.
Add hooks for some ext link stuff; add a INVALID_LINK aggregate context.
tag_defs.py -> definitions.py; more outline stuff
Add is_protocol().
Actually, they're called schemes, not protocols.
Actually, they're called URI schemes, not URL schemes.
Fix some wikilink-related tests.
Implement improved wikilink handling.
Start implementation of external links in Python.
Add a couple integration tests for ext links vs. <dl>.
Tokenizer tests for external links.
Add a couple more tests.
Fix some tests involving colons starting lines.
Fix some external links; refactor into different methods.
Alter a test that imitates strange (incorrect?) MediaWiki behavior.
Finish external links.
Some cleanup.