use heading identifiers in TOC links

reasoning: if headings have identifiers, they are probably more useful
anchors than the automatically generated anchors we build in the toc
plugin. this can happen if, for example, you use the `multimarkdown`
plugin, which inserts `id` tags for every header it encounters. this
also leverages the `headinganchors` plugin nicely.

keeps backwards-compatibility with old toc-generated #indexXhY
anchors.