Don't render if last render was less than renderThrottle ms ago

This approach works with asynch and synch usecases.
Fixes issue #170.

Author: Marco Brack

lib/node-progress.js

@@ -66,6 +66,7 @@ function ProgressBar(fmt, options) {
     head       : options.head || (options.complete || '=')
   };
   this.renderThrottle = options.renderThrottle !== 0 ? (options.renderThrottle || 16) : 0;
+  this.lastRender = -Infinity;
   this.callback = options.callback || function () {};
   this.tokens = {};
   this.lastDraw = '';
@@ -92,7 +93,7 @@ ProgressBar.prototype.tick = function(len, tokens){
 
   this.curr += len
 
-  // schedule render
+  // try to render
   this.render();
 
   // progress complete
@@ -118,6 +119,14 @@ ProgressBar.prototype.render = function (tokens) {
 
   if (!this.stream.isTTY) return;
 
+  var now = Date.now();
+  var delta = now - this.lastRender;
+  if (delta < this.renderThrottle) {
+    return;
+  } else {
+    this.lastRender = now;
+  }
+
   var ratio = this.curr / this.total;
   ratio = Math.min(Math.max(ratio, 0), 1);