Add ability to include CSS assets with media queries

[supermueller]

If a project uses css media queries for referencing CSS files, e.g. a special one for desktop computer, you have to re-write the CSS file itself to get Astrum showing your components correctly.

It would be very helpful, if you can combine the CSS reference with a media query in the data.json file as an option, e.g.:

"assets": {
    "css": [
        "/design-templates/css/all.css",
        {
            "href": "/design-templates/css/desktop.css",
            "media": "screen and (min-width: 1024px)"
        }
    ]
}

[RyanHavoc]

@supermueller Interesting idea. I'll add it to our development roadmap.