Is it possible for `rolling()` to set labels at any position within the window?
By default `rolling()` sets labels (resulting values) at the "right edge" of the window.  The right edge is where the window has the greatest index (biggest number, most recent date, etc.).  `rolling()` does have an argument to center labels, but there doesn't seem to be a way to set the label at the left edge or anywhere else.  Why can't `rolling()` have an argument that allows labels to be set at any position within the window?  Something like this:

```
> df = pd.DataFrame([1,2,3,4,5])
> df
    0
0  1
1  2
2  3
3  4
4  5 
> df.rolling(window=3,position=0).sum()
     0
0  NaN
1  NaN
2   6.0
3   9.0
4  12.0
> df.rolling(window=3,position=2).sum()
     0
0   6.0
1   9.0
2  12.0
3  NaN
4  NaN
```
