Need auto_split() pipeline

[hadley]

Autosplit should have the basic behaviour as ggplot2: i.e. it should split by every categorical variable in the layer. It should be straightforward - it's similar to by_group but it figures out which variables to split by in connect.

Then we could add branch_line() which would look like:

branch_line <- function(props, ...) {
  node(auto_split(...), mark_line(props))
}

Once we have transform_sort it might look like:

branch_line <- function(props, sort = TRUE, ...) {
  node(
    c(auto_split(), if (order) transform_sort())
    mark_line(props)
  )
}

[wch]

Implemented in 3ecea57 and 3f7e1d9.