Allow setting first day of week to any day

[realeatham]

Checklist

• [x] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [x] I have read the FAQ and my problem isn't listed.
• [x] I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• [x] This issue contains only one feature request.
• [x] I have read and understood the contribution guidelines.

Is your feature request related to a problem? Please describe. Some countries start on other days like Saturday.

Describe the solution you'd like Allow setting it to any day

Additional context

This isn't a super important thing, I don't need it myself so it's not very important and can be done later. There is a chance this is already implemented, according to the readme it isn't but I have not checked.