Ability to use a URL as the filename

[mijewe]

eg.
default: {
    options: {
        url: 'http://example.com',
        filename: 'http://example.com/assets/css/main.css',
        ...
    }
}

so that, after deployment, I wouldn't have to generate the styles again just to generate critical; I could just pull down the live styles.

[landsman]

@mijewe create variable?