use knitr::kable() tables in vignettes

[dpprdan]

We now use a knitr::kable() print method in the opencage (intro) and customise query vignettes. This is similar to the df_print: kable option usually available for the html_vignette() output, but that doesn't work for prerendered vignettes.

This also removes the date and author fields from the vignettes.