Support por custom css rules

It's not clear from the docs, there is only support for pseudo classes, like :hover?

I was looking into expressing:

```
RCSS.registerClass({
'.child': {
width: '100%'
}
```

Of course, styles could be empty for this one. But the generated css could contain the rule.
Is this supported or in the roadmap?

One rationale is that many components in React support passing a className via props, but it's common to override some styles for inner elements (such as in react-boostrap)

thanks

Start by reading the documentation and the RCSS.registerClass entry point to determine how pseudo-class rules and generated CSS are currently handled. Clarify the intended behavior for a '.child' rule with an empty style object, then define the scope and acceptance criteria for supporting custom CSS selectors.

Written by the indexing model from the issue text.