Update to CommonMark spec 0.27

[robinst]

Changelog: http://spec.commonmark.org/changelog.txt

Diff: http://spec.commonmark.org/0.27/changes.html

TODO:

• Add h2..h6 to block tag list
• Check link precedence (see Shortcut reference link with paren after it commonmark-spec#427)

[robinst]

Done in master, will be in next release (0.8.0).