Added ability to pass in a raw chunk of CSS.

I've written a small patch to allow a user to pass in a string of raw CSS. Useful if the CSS that's being analysed is coming from another source that's not on disk; this is in addition to the `stylesheets` option, but can be used exclusively as well. E.g. a sample configuration could look like:

```
var files   = ['my', 'array', 'of', 'HTML', 'files'],
    options = {
        ignore: ['#added_at_runtime'],
        raw: 'h1 { color: green }'
    };

uncss(files, options, function (output) {
    console.log(output);
});
```

Would have liked to add in unit testing for this feature but it doesn't seem like you have an automated test suite? Mocha is pretty nice. It would make adding new features like this a lot easier. Anyway, let me know what you think. :-)