Change markdown provider to kramdown
Update docs for kramdown

Kramdown expects the id and class attribute block to appear on the same
line as the header, where maruku accepted them on the line immediatley
before or after.

Move any attribute blocks that weren't already on the same line as the
corresponding header to the same line.
Remove attribute block form CHANGELOG.md

Kramdown doesn't like id values that start with a number.

Delete the attribute block from CHANGELOG.md (there is only one entry
that has a attribute block).

Note this will result in any external links to this anchor no longer
working.
Replace triple backticks with triple tilde

Kramdown doesn't support triple backtick syntax for fenced code blocks,
using tildes instead. Github accepts both.
Merge README.md conflicts