Allow options

Hey, Love the work you've done here.
I thought it would be great if you could give the user the ability to specify start_day (not just defaulting it to Sunday).
I've updated the readme to include the details but basically you can do this:
```haml
  = bootstrap_calendar month, start_day: :monday do |date|
    / Do whatever you want here!
```

This pull request is not connected to my other request.