First stab at twitter cards.

This is less a PR and more of a RFC. If you like the idea, I'll clean up the code, and add the card for the modules themselves, and make a real PR. :-)


BIG WARNING: please see if there can be escaping problems with the
<meta> tags (mostly the abstract).

Note that MetaCPAN will have to apply at
https://dev.twitter.com/form/participate-twitter-cards

There is still the module card to do, and I'm sure the elements
of the cards for the author and release could be tweaked to be
even better.

If we want to add author information to the release card, we might want
to fold the twitter_card logic in the controller, which might
not be a bad idea at all because if we do that, the TT code will
simply become


  <% IF twitter_card %>
  <% FOREACH item IN twitter_CARD %>
  <meta name="twitter:<% item.name %>" value="<% item.value %>" />
  <% END %>
  <% END %>