MEP for tags element

Hi folks,

Here's a MEP for the tags element, which should be in Mallard 1.2:

http://projectmallard.org/mep/mep0017

Experiments using tags have been floating for a while, in roughly the
form proposed here. I think the only newish thing is putting the tags
in text content, rather than an attribute. I did those almost entirely
because it makes Ducktype look super nice.

--
Shaun