Adding a keywords element

I've written a MEP for the keywords element:

http://projectmallard.org/mep/mep0014.html

There's now a MEP for everything that's planned for 1.1. My plan over
the holidays:

* Release Mallard 1.1 with full compatibility in yelp, yelp-xsl, yelp-
tools, pintail, and itstool.

* Release Cache Files 1.1 alongside Mallard 1.1.

* Prioritize enhancements to release 1.2 in roughly six months.

* Release Ducktype 1.0.

* Prioritize enhancements to release Conditionals 1.1 in roughly three
months.

* Just go ahead and call Mallard UI 1.0 final with expanders, and let
other features trickle in.

* File a slew of issues about tutorials I'd like written. That'd be a
great place for people to help out without writing specs or code.

Thanks,
Shaun