make month width and height editable via props

[raphaelpinel]

I love this project and I've been trying to implement it with a Finnish locale which have quite long 'short' months names. I tries to adjust the width via styling in props but didn't manage. Disabled Month behaved differently and the selected Month's circle was not aligned any more. I figured out there was one simple property, the width (and height + borderRadius set to its half) that allows automatically to make more space without changing any other styling. This makes it available via props.