CommonMark spec 0.28

[robinst]

Diff: http://spec.commonmark.org/0.28/changes.html
Changelog: http://spec.commonmark.org/changelog.txt

[robinst]

Released in 0.10.0: https://github.com/atlassian/commonmark-java/blob/master/CHANGELOG.md#0100---2017-09-14