Strip !important from property values

I found it wrong and annoying that cssstats counts these as two different colors:

    a { color: red }
    b { color: red !important }

`!important` shouldn't really be considered a part of the property value - it's a separate flag that affects specificity.

So created a patch that strips `!important` from property values. To my eye, the stats look much more accurate after this change.