title: MomentJS get weekNumber with variable first day of week
body: <p>I'm trying to get the weeknumer of a date, but I want it to work with a variable first day of the week. A user is able to set the first day of the week. I couldn't find a javascript solution, so I tried moment. Unfortunately, I couldn't find a way to set the first day of the week. E.g. Sunday for US, Monday for the EU, but it would be nice if it could be any other day as well. </p>