Implemented cross-references to list items ...

• Added label attribute to <item> element.
• Added cross-referencing to <item> labels.

Note this only makes sense for enumerated lists, but this isn’t checked
as there’s no clean way to do it. There’s also no check that the <item>
is inside a list element, because there are about a billion different
element variants that can be used to specify a list.

• Switched to XPath value comparison operators (eq, etc.) where appropriate. ...

• Made better use of XPath general comparison operators for lists of alternatives.
• Normalised synonyms for “center” in attributes.
• Normalised synonyms for “yes” and “no” in attributes.

• Added exercises to item-page-reference template.

Added support for boxed and numbered exercises.

Fixed permissions.

- Split cross-references and hyperlinks into separate modules.