extended_breaks should expose the "only.loose" argument of extended #99

I want to access the only.loose argument to labeling::extended when calling extended_breaks, but the function forces it to FALSE. It would make alterations to the handling of breaks more flexible if the function were updated to something like the following (adding one default function argument and updating the call to labeling::extended):

extended_breaks <- function(n = 5, ..., only.loose=FALSE) {
  function(x) {
    x <- x[is.finite(x)]
    if (length(x) == 0) {
      return(numeric())
    }

    rng <- range(x)
    labeling::extended(rng[1], rng[2], n, only.loose = only.loose, ...)
  }
}