More informative progress bars

[bgreenwell]

Default to using progress for progress bars in the plyr backend to partial (which can provide estimated time to completion, etc.):

progress_progress <- function(...) {
  pb <- NULL
  list(
    init = function(x, ...) {
      pb <<- progress_bar$new(total = x, ...)
    },
	step = function() {
      pb$tick()
    },
	term = function() NULL
  )
}