Auto detect first day of the week

[bendem]

Might I suggest to repurpose this PR for your component? For a bit less than half the world, the week starts on another day than sunday.

https://github.com/julienr114/vue-calendar-heatmap/pull/14 https://www.timeanddate.com/calendar/days/first-day-of-the-week.html

[KazzmanK]

It would be great if this could be fixed