Allow 'none' and 'currentColor' for stroke, fill

'none' and 'currentColor' are valid values for the fill and stroke
attributes
http://www.w3.org/TR/SVG/painting.html#SpecifyingPaint

'currentColor' causes the color from the 'color' attribute to be used.

'none' means than no paint is applied.
Merge pull request #67 from ewilligers/current-color

Allow 'none' and 'currentColor' for stroke, fill