Don't be too clever with how comments are highlighted.

Let the User customise those and pick sensible defaults.

Closes #4.
Link children to parent group, highlight '$' as part of the identifier.
TODOs and NOTEs must be spelt uppercase.

Quite often comments mention 'Note that...' which gets highlighted as
'phpTodo' which in turn is linked to 'Todo'. The latter has a strong
visual cue. To avoid this, the User must explicitly spell the keyword in
uppercase.