ENH:  add row and column options to margin parameter in pd.DataFrame.pivot_table
#### Is your feature request related to a problem?

Currently, the margin parameter is a simply boolean value that controls showing totals for all rows **and** columns.  However, there are times when it is either necessary or  desirable to show grand totals for only rows or only columns.   I discovered this limitation when I had a pivot_table that was constructed using cumsum() over time.  As a result, the grand totals along one of the dimensions were incorrect, and I needed to use a less-than-ideal workaround solution.  While I absolutely love pandas (still in my first year with it), Excel has a simple checkbox setting to set grand totals.

#### Describe the solution you'd like

Rather than margins=True (or False), I would suggest changing the options to the following:  Both (i.e., True in current implementation), Row, Columns, or None (i.e., False).  If desired, the rows or columns could use the standard axis=0 or axis=1 terms.  Regardless of whether or not this enhancement happens, thank you for your efforts.  Pandas has been a wonderful discovery.
