Distinguish between HTML block elements and non-element tags

Discussion at <http://talk.commonmark.org/t/drawing-a-distinction-between-html-block-elements-non-element-tags/606>

May be premature to merge.   Currently this pull request updates the spec without updating the reference implementation, but I think this distinction important.

Note also, added a pre-release label and some comments to the version number in the opening YAML block per semantic versioning idea discussed at <http://talk.commonmark.org/t/semantic-versioning/567/3>.   This label should probably change if this gets merged into jgm/master.  :-)