Add: onUpdate handle & formatting

Author: yuji

README.md

@@ -2,7 +2,7 @@
 
 Simple audio waveform visualization, depends on d3.
 
-![screenshot](https://raw.githubusercontent.com/big-data-visualization/audio-waveform/assets/screenshot.png)
+![screenshot](https://raw.githubusercontent.com/big-data-visualization/audio-waveform/master/assets/screenshot.png)
 ============
 
 ## Demo

demo/js/audio_waveform.min.js

@@ -178,7 +178,7 @@
                     return that.yScale(d)
                 }
             })
-        that.onUpdate(that, that.buf)
+        that.onUpdate.call(that, that.buf)
         return that
     }
 

dist/audio_waveform.js

@@ -182,7 +182,7 @@ fn.updateRender = function() {
                 return that.yScale(d)
             }
         })
-    that.onUpdate(that, that.buf)
+    that.onUpdate.call(that, that.buf)
     return that
 }