Introduce full support for CSS expressions

This commit introduces full support for CSS expressions such as:

- rgba(151, 134, 134, 0.2)
- rgb(255, 0, 0)
- url('http://example.org/')
- translateZ(239px)
- rotateY(230deg)
- rotateY(240deg) translateZ(288px)
- red
- transparent
- none
- etc.

For instance the following configuration file:

    {
        "color": "rgba(100, 100, 100, 0.2)",
        "border-color": "blue",
        "display": "none",
        "transform": "rotateY(240deg) translateZ(288px)",
        "text": "'some text content!'",
        "path: "some/path/to/some/file.png"
    }

Will be exported as follow in CSS:

    $color: rgba(100, 100, 100, 0.2);
    $border-color: blue;
    $display: none;
    $transform: rotateY(240deg) translateZ(288px);
    $text: "some text content!";
    $path: "some/path/to/some/file.png";

and will be exported as follow in JavaScript:

    var options = {
        "color": "rgba(100, 100, 100, 0.2)",
        "border-color": "blue",
        "display": "none",
        "transform": "rotateY(240deg) translateZ(288px)",
        "text": "some text content!",
        "path: "some/path/to/some/file.png"
    }