e.g.
.. meta::
:description: blah blah blah
:keywords lang=en: x y z
should yield
<meta name="description" content="blah blah blah">
<meta name="keywords" lang="en" content="x y z">
Also cleaned up `layout.html` a tad while at it:
* we're now properly setting the default metas via the translator so
setting them in the template is unnecessary
* there's a flag to use an html5 doctype (also has a few other
positive side-effects with respect to the metas sphinx generates in
the template)
app.add_node doesn't work right for custom translators in 1.2, since
mementos are using ``h:div`` reimplement that in the
BootstrapTranslator.
Alternative would be to monkeypatch app.add_node for Sphinx 1.2, but
let's not do that...
