Add support for twitter cards

[jhelvy]

I noted this on this issue for the {distill} package, but it would be nice if twitter cards were supported for postcard pages. Many people use them as the landing page for their site, but when shared on social media no preview image shows up. I think you might be able to some of the distill functions to insert this into the header - the metadata.R functions look promising.

[apreshill]

Hi- I use the metathis package for single Rmd docs (like this or xaringan)- I think we need a solution in distill, but wanted to make sure @seankross is aware of this package: https://pkg.garrickadenbuie.com/metathis/

[seankross]

I was not aware, thank you for letting me know.

[jhelvy]

Oh this is awesome, thanks!