[SCSS] Support interpolation in quoted strings

[gregjacobs]

Looks like interpolations aren't parsed inside quoted strings such as those provided to url(). For instance:

.my-bg {
    background: url("bg-#{$color}.gif");
}

The value node comes out as:

{
    "type": "value",
    "content": [
        {
            "type": "uri",
            "content": [
                {
                    "type": "string",
                    "content": "\"bg-#{$color}.gif\""  // <-- no parsing of interpolation
                }
            ]
        }
    ]
}

The parsing of the interpolation works correctly if the quotes are removed from inside the of url(), but of course we can't guarantee that our users will do that :)