allow dublicate headlines in toc extension

[dschick]

Adds a random hash to the id and link of each heading in the toc
extension.

[minrk]

This shouldn't be done unconditionally. It should probably only add this suffix in the event of a collision, and even then, it should probably use a counter instead of a random hash that will be different on every page load.

[jankatins]

See also here: jupyter/notebook#77

-> This breaks if one uses internal (head of the notebook to somewhere down) or external links (nbviewer) links to headlines.

[minrk]

@JanSchulz exactly. Deduplicating with counters iff there is a colision will work better than UUIDs that may or may not be stable.

[jankatins]

Counters aren't stable as well, when I add a new headline in between or before :-(

[dschick]

I havn't thought about the internal/external linking so far.
I guess that using an unique hash upon creation in the metadata of the cell is the only working option if one can handle copy/paste/split events accordingly.

Regarding the internal/external linking: if you think about LaTeX one needs some anchors/labels to do so and the actual TOC there is still build automatically from the structure/level of the headings.