Previously citations were rendered as citeproc-formatted citations
by default. Now we render them as pandoc citations, e.g. `[@item1]`,
unless the `citations` extension is disabled.
If you still want formatted citations in your markdown output,
use `pandoc -t markdown-citations`.
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.
* Depend on text.
* Expose Text.Pandoc.UTF8.
* Text.Pandoc.UTF8 now exports toString, fromString,
toStringLazy, fromStringLazy.
* These are used instead of the old utf8-string functions.
