[Fixes #3] Support `only` keyword

I added support for the `only` keyword in the AST, and allowed for it to be matched based on the `only` property in the `values` object. Here's an example:

```js

mediaquery.match('only screen and (color)', {only: true, type: 'screen', color: 1}); //=> true
mediaquery.match('only screen and (color)', {type: 'screen', color: 1}); //=> false
```

I also added unit tests for this to maintain 100% coverage.