Add hgroup (#173)

Adds the hitherto undefined element `hgroup` to html (https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/hgroup).

I assume this will also trickle down to TS/TSX and now be an allowed tag in react.

Author: thomasheartman

word_lists/html.txt

@@ -279,6 +279,7 @@ header
 heading
 height
 help
+hgroup
 hidden
 hide
 hierarchy