Fix same level arrows

[nrennie]

Adds support for same level arrows. Closes #5.

data <- tibble::tibble(
  from = c("A", "A", "A", "B", "C", "F", "C", "C", "G"),
  to = c("B", "C", "D", "E", "F", "G", "B", "D", "E")
  )
ggflowchart(data)