Stacked bar plots helper function

[zjpoh]

Create a helper function in the yellowbrick.draw module that can create a stacked bar chart from a 2D array on the specified axes. This function should have the following basic signature:

def bar_stack(data, ax=None, labels=None, ticks=None, colors=None, **kwargs):
    """
    An advanced bar chart plotting utility that can draw bar and stacked bar charts from
    data, wrapping calls to the specified matplotlib.Axes object. 

    Parameters
    ----------
    data : 2D array-like 
        The data associated with the bar chart where the columns represent each bar
        and the rows represent each stack in the bar chart. A single bar chart would 
        be a 2D array with only one row, a bar chart with three stacks per bar would 
        have a shape of (3, b).  

    ax : matplotlib.Axes, default: None
        The axes object to draw the barplot on, uses plt.gca() if not specified.

    labels : list of str, default: None
        The labels for each row in the bar stack, used to create a legend. 

    ticks : list of str, default: None
        The labels for each bar, added to the x-axis for a vertical plot, or the y-axis
        for a horizontal plot.  

    colors : array-like, default: None
        Specify the colors of each bar, each row in the stack, or every segment. 

    kwargs : dict 
        Additional keyword arguments to pass to ``ax.bar``. 
    """

This is just a base signature and should be further refined as we dig into the work. Once this helper function is created, the following visualizers should be updated to use it:

• PosTagVisualizer
• FeatureImportances
• ClassPredictionError

(@DistrictDataLabs/team-oz-maintainers please feel free to add other visualizers above as needed)

Describe the solution you'd like
A helper function to create stacked bar plot. (See #510 and #847, #771 )

Is your feature request related to a problem? Please describe.
This can be useful for ClassPredictionError, FeatureImportances, and future stacked bar plot.

[lwgray]

@zjpoh thank you for submitting this feature request.