Create css function

An interface similar to [glamor](https://github.com/threepointone/glamor) that allows to use objects and strings would be a good fit for many people.

Benefits:
- known interface, since supported by many other cssinjs libraries
- [syntax highlighting](https://github.com/cssinjs/jss/issues/591#issuecomment-402826872)
- single style sheet mode (simpler for the user)

```js
// Using objects
const className = css({color: 'red'})
```

```js
// Using templates
const className = css`color: red;`
```

## Todo

- [x] basic css function accepting style objects
- [x] composition of previously returned values from css()
- [ ] add style sheet splitting logic after a certain size
- [ ] create a babel plugin which can generate a unique id and inline it into the style object
- so that we don't generate more rules when user passes a new ref frequently
- we have no need to JSON.stringify the object to check the cache