Feature: footer with aggregate function for pivoted dataframes 
#### Problem description

It would be great to show aggregate values for pivot tables when exporting to CSV or Excel.

#### Expected Output

Something similar as in:

footer_func : {list of aggregation functions or single function}, optional

    A function or list of functions for summarizing each data column in the report. For example, ma.sum to get the sum of the column. If a list of functions is provided there must be exactly one function for each column. Be careful to ensure that the function properly handles masked values. Do not specify a function for the Date column.
 from:
http://pytseries.sourceforge.net/lib.report.html