Properly document the

`align`

argument – the function accepts “center” rather than “middle”. (#28)Fixed an issue where empty fills were not handled correctly.

The interface has now been standardized such that each implemented window function has version center-aligned by default (e.g.

`roll_mean()`

), a left-aligned version (`roll_meanl()`

), and right-aligned version (`roll_meanr()`

).Implement rolling window functions for

`mean()`

,`median()`

,`min()`

,`max()`

,`prod()`

,`sum()`

,`sd()`

and`var()`

.