Support "margin: 0 auto;"

[kristerkari]

Using margin: 0 auto; throws an error.

Here's a unit test to copy/paste:

  expect(transformCss([['margin', '0 auto']])).toEqual({
    marginTop: 0,
    marginRight: 'auto',
    marginBottom: 0,
    marginLeft: 'auto',
  })

[jacobp100]

Good catch! We’ll need to support top/bottom as well, so it should be pretty easy to update in transforms/index.js