Add optional EXTANCHORS extension that uses all TextNodes of a header node for Anchor generation

[vsch]

This is not the Final Solution with fancy resolution of collisions. It is simply the ability to create anchors for headers that are currently skipped, making the document a bit inconsistent.

Fix with tests. Anyone wanting to make it more sophisticated please do so. I'm swamped and this is all I could do on a limited budget. With simple tests, PR right around the corner.