This makes it possible to generate UL or OL lists as well as the
current divs and spans.
This feature is clearly necessary but I'm not very happy with it. It
feels as though the syntax should be modifying a UL tag to specify the
extra information required to generate the list, rather than turning
the list widget to indirectly specify it's elements.
What we have at the moment isn't really the same as TiddlyWiki
classic's shadow tiddlers, it's a much simpler system for excluding
tiddlers. We'll use the term "shadow" instead to refer to the way that
tiddlers in plugins behave, which is exactly like TiddlyWiki classic's
shadow tiddlers.
