WCAG 2.1 Accessibility features

[r-leyshon]

Issues found in testing .rmds on WAVE:

• [ ] author & date are specified as h4, should be h2 with styling in css.
• [ ] no page regions specified. Suggest replacing toc <divs> with <nav>, appropriate use of <main> and so on.
• [ ] no html lang attribute is specified. This can be corrected with lang: "en" in the YAML.
• [ ] very low contrast in chunk output - affects code and code comments. Also applies to the Bootswatch readable theme.

Running {rmarkdown} v2.9

[r-leyshon]

For anyone else that find these a priority, I've written a package that helps to avoid some of these 'gotchas' - accessrmd. 1.0.0 now on CRAN.