Project: svg-solar
Task (challenging): Define gradient color format:`(linearGradient|radialGradient): offset stop-color stop-opacity[, offset stop-color stop-opacity]...`, `stop-opacity` is optional, default value is 1. When data color is gradient, append systemDefs with a gradient element whose color direction is downward linearGradient or default radialGradient, and children elements attributes come from parsed color values. Replace the fill of star or planet with `url(#gradient-{name})`.