hgroup removed from HTML 5.1

The following commits remove hgroup from HTML 5.1, include it in the list
of obsoleted features[1] and adds advice in Common idioms without dedicated
elements for marking up subheadings, etc.[2]

https://github.com/w3c/html/commit/5db030539ce491f5ca1d855cc262f135d9e7b19b
https://github.com/w3c/html/commit/aa9445f1f110e2de40ac0e0229c48bd708c1bf0c
https://github.com/w3c/html/commit/3c6cb3840a9cf25d83d8205ef53cc592201ab51c

please review, feedback welcome. I will make the same commits to HTML5 CR
after feedback.


[1]
http://www.w3.org/html/wg/drafts/html/master/obsolete.html#non-conforming-features
[2] http://www.w3.org/html/wg/drafts/html/master/common-idioms.html#sub-head

--

Regards

SteveF
HTML 5.1 <http://www.w3.org/html/wg/drafts/html/master/>