Is there any ways to get style for pseudo-class?

* `jest-glamor-react` version: v4.2.1
* `node` version: v8.9.4
* `npm` (or `yarn`) version: v5.6.0

Relevant code or config

```javascript
const TestComp = glamorous.div({
'::after': {
backgroundColor: 'white',
},
});

it('should have white background', () => {
const component = enzyme.shallow();
expect(component).toHaveStyleRule(/* What should I do ??? */);
});
```

What you did:
Try to test pseudo-class style

What happened:
No way to test it

Problem description:

Suggested solution:
Add some matchers? or add functions to the matcher?

Start from the shown toHaveStyleRule matcher call and the glamorous.div example using ::after. Trace how style rules are currently asserted, then define and verify a pseudo-class or pseudo-element assertion whose expected result is demonstrated by a passing test for the white background.

Written by the indexing model from the issue text.