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.
Still TODO:
- documentation in README
- add default.asciidoc to templates/
- lists
- tables
- proper escaping
- footnotes with blank lines - print separately at end?
currently they are just ignored.
- fix header (date gives weird result on pandoc README)
