ENH: Add hue functionality to df.hist() 
The following command:

    df.hist(layout=(4, 4), figsize=(15,10), bins=30)

produces a grid of histograms. I would like to group the data by a column value and plot the corresponding histograms in the same plot with different colors. Unfortunately, the `by=` argument generates subplots. And plots all the data regardless the scale in as many subplots as unique values in the dataframe. 

I wonder if either an argument `hue=` as used in seaborn could be added so to produce the desired result. 