Possible Metropolis theme improvements

[eddelbuettel]

In no particular order, and in comparison to the demo.tex-based demo.pdf of the LaTeX variant:

• No standout pages :-( which is a bummer and possibly hard to adjust due to how we get from new Markdown slide entry to \begin{frame} ... \end{frame}
• Table of contents page has no header
• References page has no header (but we changed things anyway with natbib use)
• Metropolis has a bazillion (nice) options which we should support from YAML metadata where useful and sensible

[cschwem2er]

Hi there, just wanted to thank you for this package, it is awesome! Implementing standout pages at some point would make it even better :)