Allow style keys to be strings

[maksimsco]

In general it works, however some values are incorrectly post-fixed with "px".

const style = Stilr.create({
    header: {
        'z-index': 9999
    }
});

This style will be converted to:

.header{z-index:9999px}

[chriskjaer]

This would actually be a react issue since Stilr uses React's own css handler (https://github.com/facebook/react/blob/master/src/renderers/dom/shared/CSSPropertyOperations.js) internally to convert js objects to css strings.

Also, I would always recommend using camelCase. Since it's the normal way to interact with style properties in plain old javascript. e.g.

document.querySelector('.button').style.zIndex = 9999;
// or
document.querySelector('.foo').style.backgroundColor = 'tomato';

...and it saves you 3 key strokes ;)