The option was buggy. Rather than try to fix it, it is being removed.
This feeture should exist as a seperate extension. Fixes #390.

Fixes #386. I'm doing this against my better judgement. The only reason is
that I'm using the HTML format suggested by the HTML5 Spec and will simply
not consider any alternate output. If a JavaScript library requires something
else, to bad. I don't care. That library should support the format
suggested by the spec or I'm not interested in it. If you want something else
then you can create your own extension which does whatever you want.

Use the Table of Contents Extension instead. The HeaderId Extension will
raise a PendingDeprecationWarning.

The last few features of the HeaderID extension were mirgrated to TOC
including the baselevel and separator config options. Also, the
marker config option of TOC can be set to an empty string to disable
searching for a marker.

The `slugify`, `unique` and `stashedHTML2text` functions are now defined
in the TOC extension in preperation for the HeaderId extension being
removed. All coresponding tests are now run against the TOC Extension.

The meta-data support of the HeaderId Extension was not migrated and no plan
exists to make that migration. The `forceid` config option makes no sense in
the TOC Extension and the only other config setting supported by meta-data
was the `header_level`. However, as that depends on the template, it makes
more sense to not be defined at the document level.

Fixes #375. Explains the difference between returning None and returning a modified root element. Also makes the docs more consistent with the doc strings in the code.

Correct the documentation for the slugify function