Update to CommonMark spec 0.26

[robinst]

Changes (from here):

• empty list items can no longer interrupt a paragraph; this resolves an ambiguity with setext headers
• ordered lists can interrupt a paragraph only when beginning with 1
• the two-blank-lines-breaks-out-of-lists rule has been removed
• the spec for emphasis and strong emphasis has been refined to give more intuitive results in some cases
• tabs can be used after the # in an ATX header and between the markers in a thematic break

Spec changelog: http://spec.commonmark.org/changelog.txt
Spec diff: http://spec.commonmark.org/0.26/changes.html

[robinst]

Just merged the PR for this, will release soon.

[robinst]

Released 0.6.0 which includes these changes: https://github.com/atlassian/commonmark-java/releases/tag/commonmark-parent-0.6.0