Dark (FT) Roboto RMarkdown template

[mevers]

Hi @hrbrmstr

I've created a dark (FT-based) Roboto R markdown template called FT (based on the existing ipsum HTML template); perhaps this is useful for others as well so I've submitted a PR.

It only works for HTML output (a dark PDF theme doesn't really make sense IMO).

Upon installation, it should show up as a template

knitting then gives