title: Apply function on cumulative values of pandas series
body: <p>Is there an equivalent of <code>rolling_apply</code> in pandas that applies function to the cumulative values of a series rather than the rolling values?  I realize <code>cumsum</code>, <code>cumprod</code>, <code>cummax</code>, and <code>cummin</code> exist, but I'd like to apply a custom function.</p>