css string support

Hi @mattdesl!
I’ve noticed that sometimes it is useful to pass css string directly instead of css object:

``` js
css(el, `
font-size: 12px;
font-family: ${fontFamily};
...
`);
```

Because it alows for avoiding css-in-js notation, which is copy-paste compatible and in other ways handy.

Is there any sense to add **css.set(element, string)** clause?

Start by locating the public css API and comparing the proposed css.set(element, string) entry point with the existing css(element, object) usage shown in the issue. Define the expected string-input behavior and verify the example’s result before considering the request complete.

Written by the indexing model from the issue text.