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.
Previously the textile reader and writer incorrectly implented
RST-style autolinks for URLs and email addresses.
This has been fixed. Now an autolink is done this way:
"$":http://myurl.com
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.
