Use the Locale on date-fns functions

[Tom Chen]

Issue

#38 is a good demonstration for this bug. If I change locale to the one that has Monday as their first day of the week, the calendar will not respect that. Eg. Italian locale https://github.com/date-fns/date-fns/blob/v2.25.0/src/locale/it/index.js

Has an options.weekStartsOn = 1. With #38, it gives:

Where Dom (domenica) is Sunday in Italian and Lun (lunedì) is Monday in Italina

Solution

Whenever date-fns functions are used, they should given the locale object.
https://date-fns.org/v2.28.0/docs/Locale

I will see if I could submit a PR for this. Thank you