added onerror option and test

package.json

@@ -1,5 +1,5 @@
 {
-  "version": "1.4.1",
+  "version": "1.4.2",
   "name": "twemoji",
   "license": ["MIT", "CC-BY-4.0"],
   "description": "A Unicode standard based way to implement emoji across all platforms.",

test.js

@@ -535,4 +535,22 @@ wru.test([{
       wru.assert('nothing to do here');
     }
   }
+}, {
+  name: 'using a different onerror',
+  test: function () {
+    var Image = window.Image;
+    window.Image = function () {
+      var self = new Image;
+      setTimeout(function () {
+        window.Image = Image;
+        self.onerror();
+      }, 10);
+      return self;
+    };
+    var div = document.createElement('div');
+    div.innerHTML = '5\ufe0f\u20e3';
+    twemoji.parse(div, {onerror: wru.async(function () {
+      wru.assert('OK');
+    })});
+  }
 }]);

twemoji-generator.js

@@ -664,7 +664,7 @@ function createTwemoji(re) {
               );
               if (src) {
                 img = new Image();
-                img.onerror = twemoji.onerror;
+                img.onerror = options.onerror;
                 img.setAttribute('draggable', 'false');
                 attrib = options.attributes(icon, variant);
                 for (attrname in attrib) {
@@ -828,7 +828,8 @@ function createTwemoji(re) {
           base:       typeof how.base === 'string' ? how.base : twemoji.base,
           ext:        how.ext || twemoji.ext,
           size:       how.folder || toSizeSquaredAsset(how.size || twemoji.size),
-          className:  how.className || twemoji.className
+          className:  how.className || twemoji.className,
+          onerror:    how.onerror || twemoji.onerror
         });
       }