kupu should not accept all named anchors as Silva index items

Right now, Kupu assumes all named anchors ('<a name="foo"> in HTML) are to be
index items. This is however rarely the case when you're copying and pasting
HTML from foreign sites -- the named anchors there typically should *not* become
Silva index items.

Kupu should distinguish Silva index items from ordinary named anchors somehow.
One way to do this is to add a special kupu attribute to Silva-generated HTML
for Kupu, so that Kupu can recognize these as actual index items.