Hi!

Can you add toStartOfWeek function?

Behavior of this function is similar to toStartOfMonth,
but I want that function returns truncated to monday's date.

example :

toStartOfWeek(2016-06-28, 0) -> 2016-06-26
toStartOfWeek(2016-06-23, 1) -> 2016-06-20

here is first argument is a date
second argument is a first day week (sunday, monday ...)