Add option for "impartial binning" for histograms

[rpruim]

From @rpruim on July 26, 2016 12:40

Traditionally (i.e., in hist()) one must choose between left-leaning and right-leaning binning with right = FALSE or right = TRUE. A more balanced option would be to allow right = 0.5. Under this option, data values exactly on a bin border would count has 0.5 in the bin to the left and 0.5 in the bin to the right.

Naive implementation is easy:

newcounts <- 
  right       * hist(..., right =  TRUE)$counts + 
  (1 - right) * hist(..., right = FALSE)$counts

This would work for other values of right a well, although it is less clear that other values are useful. Setting right = TRUE or right = FALSE still works as before (although less efficiently).

Copied from original issue: hadley/ggplot2#1680