Backlinks

[goerz]

Via @odow: It would be nice if the bibliography block could provide backlinks to where the reference was cited in the document.

This would likely require a fourth pass:

1. Collect Citations
2. Render Bibliography
3. Expand citations to link to the items in the bibliography
4. Add backlinks to the rendered bibliography ←

As of #46, the "rendered" bibliography is not longer a raw HTML block, but a special markdown subtree, which is only converted to HTML (or LaTeX) during the final Writer stage. That would seem to make this feasible.