Add a basic cx api so that users can merge emotion and custom classes

[tkh44]

Add a basic cx api so that users can merge emotion and custom classes similar to the classnames library without the quirks of selectivity.

test('all types', () => {
  const cls1 = css`
    font-size: 20px;
    background: green;
  `
  const cls2 = css`
    font-size: 20px;
    background: blue;
  `

  const cls3 = css`
    font-size: 20px;
    background: darkorange;
  `

  const cls4 = css`
    font-size: 20px;
    background: darkgreen;
  `

  const foo = true
  const bar = false

  const tree = renderer
    .create(
      <div
        className={cx(
          { [cls1]: foo },
          { [cls2]: bar },
          () => 'modal',
          'XxX420_no_scopeXxX',
          [cls3, cls4]
        )}
      />
    )
    .toJSON()
  expect(tree).toMatchSnapshot()
})

.glamor-0 {
  font-size: 20px;
  background: green;
  font-size: 20px;
  background: darkorange;
  font-size: 20px;
  background: darkgreen;
}

<div
  className=" modal XxX420_no_scopeXxX glamor-0"
/>

Checklist:

• Documentation
• Tests
• Code complete

[tkh44]

This is a nice little bonus

test('fun fun functions', () => {
  const cls1 = css`
    font-size: 20px;
    background: green;
  `
  const cls2 = css`
    font-size: 20px;
    background: blue;
  `

  const cls3 = css`
    font-size: 20px;
    background: darkorange;
  `

  const cls4 = css`
    font-size: 20px;
    background: darkgreen;
  `

  const tree = renderer
    .create(
      <div
        className={cx(() => () => [
          () => [cls1, false && cls2, 'modal'],
          () => [() => [cls3, () => ({ [cls4]: true }), 'profile']]
        ])}
      />
    )
    .toJSON()
})

[codecov]

Codecov Report

Merging #397 into master will increase coverage by 0.07%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
packages/emotion/src/index.js	100% <100%> (ø)	⬆️