Add weeknumbers to calendar

[nonpop]

Hi, this preliminary PR adds week numbers to the calendar view. I can polish the code and add tests later, maybe even make it a minor release instead of major, but before any of that I'd like to know if you would be interested in merging such a feature in the first place.

[abadi199]

Hi @nonpop
Thanks for the PR. I think it's valuable to be able to show week number to the calendar, so I will definitely merge this once it's done. I don't have time to review the code right now, but at first glance, it looks like you're using different version of elm-format with what I'm using. Please upgrade the code to use elm-format@exp to keep the style consistent.

Thanks again for your contribution.