Move litr::document() to litr::render()?

[jacobbien]

The arguments for making the move are

• we always need it to be run, so why leave it up to the user
• we want the Rmd to be as uncluttered as possible

[jacobbien]

A downside is if devtools::build() or devtools::install() or devtools::check() or devtools::test() are in the Rmd, we would need to include litr::document() to be run first. However, a user can simply add litr::document() before these other commands if so desired (in which case litr::document() would be run twice).