Add classNames

[luixo]

@wojtekmaj

Hello,
Following the recently discussed theme support for the package, I decided to make a draft for what it could look like.
This one lets a complete restyle of the calendar in a variety of wide-spread solutions (specifically, tailwindcss and css-in-js that let user retrieve classnames).

First of all, let me know if you're not up for this change in any way.

In case you are, there are a few points to discuss:

• specific property names for classNames
• nested classNames - I did what felt like a good compromise, but it can be more more nested / less nested
• should className get deprecated in case this PR gets merged?
• should package styling move towards css variables?
• should tileClassName be the only function to generate the className? can we make it par with the inner class name function generator (getTileClassName) and let there be multiple tile className generators?

If you approve the concept, I'll do testing and some examples.