export CustomizableCalendarDay styles, add story

[a15n]

Previously if you wanted to update a single CSS style from CustomizableCalendarDay you'd need to (1) make the singular change and (2) duplicate all of the original CSS.

With this change we can import the style definitions and update the single element using es6 spread operator or update.

// before
const newDefaultStyles = {
  color: 'blue', // what we want to change
  // what we need to keep the same
  border: `1px solid ${color.core.borderLight}`,
  background: color.background,
  hover: {
    background: color.core.borderLight,
    border: `1px double ${color.core.borderLight}`,
    color: 'inherit',
  },
}


// after
import CustomizableCalendarDay, { defaultStyles, selectedStyles } from '../src/components/CustomizableCalendarDay';

const newDefaultStyles = {
  ...defaultStyles,
  color: 'blue',
};

[a15n]

couldn't think of a good intermediate name so I just execute everything inline

[majapw]

That seems reasonable to me

[coveralls]

Coverage decreased (-0.05%) to 86.488% when pulling c28737d on a15n:as--export-CCD-styles into 7a03e4e on airbnb:master.

[ljharb]

i'm not sure this library needs calling out in an example

[majapw]

I think in your description, you probably meant

const newDefaultStyles = {
  ...defaultStyles,
  color: 'blue',
};

and not the other way around, right? :P

In any case, looks great to me!

[majapw]

That seems reasonable to me

[a15n]

Thanks for the review. Ready to merge