Modifier classes

**Do you want to request a *feature* or report a *bug*?**

feature

**What is the expected behavior?**

To have the same modifiers syntax as in [`astroturf`](https://github.com/4Catalyzer/astroturf)

```js
const Button = styled('button')`
color: black;
border: 1px solid black;
background-color: white;

&.primary {
color: blue;
border: 1px solid blue;
}

&.color-green {
color: green;
}
`;

```

Compare to `${ }` interpolation it keeps CSS syntax. CSS highlight and autocomplete works.