nbextensions/usability/toc2/: Fixed problems with skipped heading levels, nested TOC lists

[slonik-az]

• Table of Contents was empty when the first heading is <h2> or higher.
• TOC had incorrectly numbered labels when some heading levels are skipped over.
• Make calculations of numbered labels more robust.
• Minor code cleanup

[jfbercher]

Hi @slonik-az.
I have checked the code and tested your modifications. This is a nice work that largely improves over what I did. This is very useful -- I was myself annoyed to be forced to begin the notebook with a level-1 heading. Thanks.
@juhasch, I think that we can merge as is.

[jankatins]

BTW: IMO it would be nice to a) add IDs so that you wouldn't need to add them on your own if you display H1 from a kernel and b) to check that a ID wasn't already included in the list if if it was, do some magic that the links work anyways...

[The last is a problem when including something like this: "analysis 1": "plots", "stats"; analysis 2: "plots", "stats" -> the links for both "stats" all go to the first one because the notebook does not render unique IDs -> https://github.com/jupyter/notebook/issues/77]

[jfbercher]

I finally understood what is the actual problem... I did an essay to address it (since we number headings, we can try to use this number as a kind of unique marker). Of course external links may fail when toc changes but I suggest a way to address this too -- see #488

[slonik-az]

@jfbercher, @juhasch:
Thanks for merging my patch into master! toc2 is a great extension which makes notebook navigation so much more fun. Making it more robust benefits everybody.
--Leo