add clipping js code

lib/extract/extractStroke.js

@@ -0,0 +1,55 @@
+import rgba from '../rgba';
+import extractBrush from './extractBrush';
+let separator = /\s*,\s*/;
+
+const caps = {
+    butt: 0,
+    square: 2,
+    round: 1
+};
+
+const joins = {
+    miter: 0,
+    bevel: 2,
+    round: 1
+};
+
+function strokeFilter(props) {
+    if (!props.stroke && !props.strokeLinecap && !props.strokeOpacity &&
+        !props.strokeLinejoin && !props.strokeDasharray && !props.strokeWidth) {
+        return null;
+    }
+
+    let strokeDasharray = props.strokeDasharray;
+
+    if (typeof strokeDasharray === 'string') {
+        strokeDasharray = strokeDasharray.split(separator).map(dash => +dash);
+    }
+
+    return {
+        stroke: rgba(props.stroke, props.strokeOpacity).rgbaArray(),
+        strokeLinecap: caps[props.strokeLinecap] || 2,
+        strokeLinejoin: joins[props.strokeLinejoin] || 0,
+        strokeDash: strokeDasharray || null,
+        strokeWidth: +props.strokeWidth || 1
+    };
+}
+
+export default function(props) {
+    let strokeProps = strokeFilter(props);
+    if (!strokeProps) {
+        return {};
+    }
+
+    let {stroke} = strokeProps;
+    return {
+        ...strokeProps,
+        stroke: stroke ? [stroke[0] / 255, stroke[1] / 255, stroke[2] / 255, stroke[3]] : null
+    };
+
+    // TODO: implement brush on stroke prop
+    //return {
+    //    ...strokeProps,
+    //    stroke: extractBrush(strokeProps.stroke, props)
+    //};
+}