Add support for Graphite web holtWintersConfidenceArea() function

[carrieedwards]

Is your feature request related to a problem? Please describe.
The holtWintersConfidenceArea() function is implemented in Graphite web, but is missing from CarbonAPI. In order to enable users to be able to use the holtWintersConfidenceArea() function, it needs to be implemented in CarbonAPI.

Describe the solution you'd like
The holtWintersConfidenceArea() function should be implemented in CarbonAPI, with the same functionality as the Graphite web function. The description of the function is as follows:

holtWintersConfidenceArea(seriesList, delta=3, bootstrapInterval='7d', seasonality='1d')

Performs a Holt-Winters forecast using the series as input data and plots the area between the upper and lower bands of the predicted forecast deviations.

Additional context
Tests should be added to verify that the functionality matches that of the Graphite web holtWintersConfidenceArea() function.