Added optional argument to getWeeksForMonth to set start day of week

[nimzco]

First step to tackle: https://github.com/Shopify/polaris-react/issues/1738

Context

In most of western countries calendar starts on Monday. In some other countries, it can even start on Saturday.

What

• Adding extra parameter to getWeeksForMonth() to be able to pass the start week day.
• By default, if nothing is passed the behaviour stays the same.

[WillsonSmith]

I'd split out the config and package json updates to another PR

[nimzco]

You're right: ✨https://github.com/Shopify/javascript-utilities/pull/35

[nimzco]

I will keep all of the changes in different commit in this PR because I'm running into weird problem having it separated in another PR...

[nimzco]

Can you folks have a look today? 🚀

[nimzco]

@tzvipm @TheMallen Do you have time to have a look today?

[nimzco]

Thanks Mal!