Add support for schema.org attributes: itemscope, itemtype, itemprop

[terjesb]

http://schema.org/docs/gs.html

[sophiebits]

Fixed by #1491.

[rektide]

Microdata microdata microdata microdata microdata!

Hopefully I can now find this issue when I look for it. These attributes are all microdata attributes, so I wanted this issue to be find-able online when searching for the spec they are from. Commenting here just to add that searchability to this issue.

Awh what the heck: rdfa. Even though RDFA is not implemented in React, some kind of useful result when searching for it would be good too.

Oh, did I mention? Microdata.

[obedparla]

Microdata!

[gaearon]

React 16 supports arbitrary string attributes now.