Previously header ids were autogenerated by the writers.
Now they are generated (unless supplied explicitly) in the
markdown parser, if the `header_identifiers` extension is
selected.
In addition, the textile reader now supports id attributes on
headers.
Additional related changes:
* URLs in Code in autolinks now use class "url".
* Require highlighting-kate 0.2.8.2, which omits the final <br/> tag,
essential for inline code.
The old TeX, HtmlInline and RawHtml elements have been removed
and replaced by generic RawInline and RawBlock elements.
All modules updated to use the new raw elements.
