[core] Return NONE for invalid color strings in `Color::parse()`

[anandthakker]

Color::parse (via CSSColorParser::parse()) defaults to black for invalid / unparseable color strings. This prevents us from correctly evaluating property functions for color values in the case where the property value is an invalid color string -- see mapbox/mapbox-gl-js#4150

[jfirebaugh]

I'd be fine with modifying our vendored copy of CSSColorParser to support this.

[kkaefer]

Transferred the repo to https://github.com/mapbox/css-color-parser-cpp