Added reviver method in parser

The optional method is used when asking fabric to
load an SVG file and parse it, instantiating fabric objects.
After each fabric object is created, reviver(ele, obj) is
called so you can parse our custom attributes, or do
anything else custom.

src/parser.js

@@ -320,8 +320,9 @@
    * @param {Array} elements Array of elements to parse
    * @param {Function} callback Being passed an array of fabric instances (transformed from SVG elements)
    * @param {Object} options Options object
+   * @param {Function} [reviver] Method for further pasring of SVG elements, called after each fabric object created.
    */
-  function parseElements(elements, callback, options) {
+  function parseElements(elements, callback, options, reviver) {
     var instances = Array(elements.length), i = elements.length;
 
     function checkIfDone() {
@@ -340,15 +341,18 @@
       if (klass && klass.fromElement) {
         try {
           if (klass.async) {
-            klass.fromElement(el, (function(index) {
+            klass.fromElement(el, (function(index, el) {
               return function(obj) {
+				reviver && reviver(el, obj);
                 instances.splice(index, 0, obj);
                 checkIfDone();
               };
             })(index), options);
           }
           else {
-            instances.splice(index, 0, klass.fromElement(el, options));
+			var obj = klass.fromElement(el, options);
+			reviver && reviver(el, obj);
+            instances.splice(index, 0, obj);
             checkIfDone();
           }
         }
@@ -438,6 +442,7 @@
    * @method parseSVGDocument
    * @param {SVGDocument} doc SVG document to parse
    * @param {Function} callback Callback to call when parsing is finished; It's being passed an array of elements (parsed from a document).
+   * @param {Function} [reviver] Method for further pasring of SVG elements, called after each fabric object created.
    */
   fabric.parseSVGDocument = (function() {
 
@@ -467,7 +472,7 @@
       return false;
     }
 
-    return function(doc, callback) {
+    return function(doc, callback, reviver) {
       if (!doc) return;
 
       var startTime = new Date(),
@@ -525,7 +530,7 @@
         if (callback) {
           callback(instances, options);
         }
-      }, clone(options));
+      }, clone(options), reviver);
     };
   })();
 
@@ -569,8 +574,9 @@
     * @method loadSVGFromURL
     * @param {String} url
     * @param {Function} callback
+    * @param {Function} [reviver] Method for further pasring of SVG elements, called after each fabric object created.
     */
-   function loadSVGFromURL(url, callback) {
+   function loadSVGFromURL(url, callback, reviver) {
 
      url = url.replace(/^\n\s*/, '').trim();
 
@@ -606,7 +612,7 @@
            options: options
          });
          callback(results, options);
-       });
+       }, reviver);
      }
    }
 
@@ -630,8 +636,9 @@
     * @method loadSVGFromString
     * @param {String} string
     * @param {Function} callback
+    * @param {Function} [reviver] Method for further pasring of SVG elements, called after each fabric object created.
     */
-  function loadSVGFromString(string, callback) {
+  function loadSVGFromString(string, callback, reviver) {
     string = string.trim();
     var doc;
     if (typeof DOMParser !== 'undefined') {
@@ -649,7 +656,7 @@
 
     fabric.parseSVGDocument(doc.documentElement, function (results, options) {
       callback(results, options);
-    });
+    }, reviver);
   }
 
   function createSVGFontFacesMarkup(objects) {