Styling for google-calendar-busy-now

[benhjt]

Would it be possible to add some custom properties and mixins to style the contents of the google-calendar-busy-now element?

I was thinking --google-calendar-busy-now-busy-background-color, --google-calendar-busy-now-free-background-color, --google-calendar-busy-now-na-background-color, --google-calendar-busy-now-text-color and a mixin of --google-calendar-busy-now.

I'm happy to do a pull request on this if you agree on these changes.