Expose num_threads for the vroom backend

[hsbadr]

The 2nd edition of readr uses vroom as a backend, which by default utilizes parallel::detectCores() unless the user sets num_threads argument or VROOM_THREADS environment variable. This can overwhelm the resources when reading multiple files in parallel. So, it would be useful to expose num_threads, defaulting to 1L or vroom::vroom_threads().