Keyrings and usethis::edit_r_environ()

[etam4260]

In vignettes/Setup.Rmd, it could be useful to encourage users to set their environment key via usethis::edit_r_environ(). This would persist across sessions. Working with the renviron file seems cleaner than all the regex and string manipulation in hud_set_key(), though the key would be exposed to any package in R. In the medium term, it would be nice to also offer the option to use a keyring.

-rtaph