Add support for new syntax

[panec]

item_: (
        background-color : (
            default                           : #FFF,
            ":nth-child(1)"                   : invert( rgba( #F00, 1 ) ),
            ":nth-child(2)"                   : invert( rgba( #0F0, 1 ) ),
            ":nth-child(3)"                   : invert( rgba( #00F, 1 ) ),
            ":nth-child(n+4):nth-child(-n+7)" : invert( rgba( #F0F, 1 ) ),
        ),

        clear : (
            default            : null,
            ":nth-child(3n+4)" : (
                default : null,
                md      : left,
            ),
            ":nth-child(2n+3)": (
                default : left,
                md      : null,
            ),
        ),
        float : left,
        width : (
            default : 20%,
            xxs     : 50%,
            xs      : 33.3%,
            sm      : 25%,
            md      : 20%,
        )
    )