implement non-interactive flow for create_if_needed()

[pachadotdev]

implements #1154

I think the correct approach in non-interactive mode should be:

1. check if the file/dir exists
2. if it does not exist, create it
3. if it exists, refuse to overwrite and print a warning except is a new parameter is set to overwrite = T
4. besides it, allow to hide warnings with an extra parameter (i.e., for testthat when we test overwriting)