Support custom labels in TOC.

[waylan]

When the Table of Contents Extension is used in conjunction with the Attribute Lists Extension and a data-toc-label attribute is defined on a header, the content of the data-toc-label attribute is now used as the content of the Table of Contents item for that header.

[facelessuser]

Interesting workaround for sane tocs with complex headers.

[waylan]

Yeah, I've had the idea bouncing around for a while, but was thinking of using the title attribute. I wasn't crazy about it though as there is a legitimate use for titles and they are potentially visible to end users (and screen readers). Today it occurred to me that we could use a data-* attribute. And then I had it working with only one line of code. Writing the documentation was the hardest part.

We should probably still properly fix #660, although this would serve as a workaround in the meantime. Note sure if there is anything else we could do for #699 though.

[waylan]

The last commit (94b9c31) just added a another (unrelated) feature to TOC:

A new toc_tokens attribute has been added to the Markdown class, which contains the raw tokens used to build the Table of Contents. Users can use this to build their own custom Table of Contents rather than needing to parse the HTML available on the toc attribute of the Markdown class.