Add startOffset support

lib/attributes.js

@@ -99,7 +99,8 @@ const TextAttributes = merge({
 }, RenderableAttributes);
 
 const TextPathAttributes = merge({
-    href: true
+    href: true,
+    startOffset: true
 }, RenderableAttributes);
 
 const TSpanAttibutes = merge({

lib/extract/extractText.js

@@ -88,7 +88,8 @@ export default function(props, container) {
         y,
         dx,
         dy,
-        textAnchor
+        textAnchor,
+        startOffset
     } = props;
 
 
@@ -121,6 +122,7 @@ export default function(props, container) {
         content,
         deltaX,
         deltaY,
+        startOffset: (startOffset || 0).toString(),
         positionX: _.isNil(x) ? null : x,
         positionY: _.isNil(y) ? null : y
     }