edit_r_profile("user") should test R_PROFILE_USER first

[heavywatal]

Currently, edit_r_profile() and edit_r_environ() always try to create/edit a file directly on the user's home directory regardless of R_PROFILE_USER and R_ENVIRON_USER.

Sys.setenv(R_PROFILE_USER = "/tmp/path/to/.Rprofile")
Sys.getenv('R_PROFILE_USER')
#> [1] "/tmp/path/to/.Rprofile"
usethis::edit_r_profile()
#> ● Edit /Users/watal/.Rprofile
#> ● Restart R for changes to take effect

[hadley]

This needs a little thought as the existing functions need some refactoring.

Possibly helpful snippet:

envvar <- function(x) {
  env <- Sys.getenv("R_ENVIRON_USER", unset = "")
  if (identical(env, "")) {
    NULL
  } else {
    env
  }
}