Subsets of the Superset

Issue raised: https://github.com/w3c/WebID/issues/21

Once the superset specification has been drafted, we will also need subset
specs as outlined in #17 <https://github.com/w3c/WebID/issues/17>

Which ones should be selected?

Here's my votes:

+1 JSON-LD
+1 Turtle

-1 RDFa
-1 anything that mandates XHTML

+0 everything else

The reasoning behind this is to align with current adoption, popularity
with web developers, backwards compatibility, and areas that have a chance
of achieving broad adoption and growth.

Happy to hear other opinions on this.