Feature Request

I would like to be able to specify the day of the week in the floor and ceil functions, e.g.

firstday = month.floor('week',week_start=7)

For now I am doing this as a workaround:

firstday,_ = month.span('week',week_start=7)

which is essentially all that floor is doing anyway, so it looks like it should be possible to just pass along a **kwargs in both floor and ceil so that they also get all of the other optional functionality of span.