I want to do some things that I think are easiest accomplished
by allowing me to add arbitrary HTML to be embedded on all pages
in the site. Specifically, I want to add meta tags to the top of
the page so that it renders pretty-like in things like Twitter,
and I want to add Piwik tracking to the bottom of the page.

So how do I do that?

I could write a whole new template for the site, but I suspect
that there's a more modular approach that is advised. And if you
have ideas of totally different ways do do this, do tell.

Thanks