HTML export: Add point-and-click link

[uliska]

This is an additional suggestion to #52, but I want to have that independently (i.e not blocking a merge).

Exported annotations should (optionally) be able to carry a link back to the input location (i.e. the "point-and-click" link). An annotation viewer (and editor) has to be able to locate the annotation in the input file, so we need that link when developing such a GUI.

I'm not sure where this link should be applied. It could be directly to the annotation div to have the whole div element act as a link (might be most consistent with current web design paradigms). Or an explicit textual link could be inserted somewhere.