Create header for syntax highlighting stuff.
Track the order that source mode is set in.
Stack the tagged source modes.
Support source code tagging for lists.
Support source code tagging for paragraphs.
Support source code tagging for sections.
Document the new source mode changes.
Only allow phrase templates in attribute values.
New parser for attribute value template calls.
Disallow empty 1.7 attribute values

Only in the parser, an empty template could still be used. Will need to
validate attributes somewhere else.
Allow template calls in anchors, roles and includes.

Actually a subtle change, as includes and anchors could previously
contain spaces. Possibly should still alow that, although I expect it's
never used.
Update 1.7 note about attribute templates.