Linking to tags by id attributes

[max-heller]

<a id="test">some text here</a>
[test link](test)

is converted to

some text here\href{test}{test link}

but there should be a link target defined with the tag's id ("test").

Pandoc deliberately does not try to parse raw HTML, so this needs either a full-fledged HTML parsing solution (#44) or a hacky, best-effort solution