Add new function goToStepNumber

This allows to go to a specific step with the specific `data-step`
attribute. This differs from `goToStep` in the way that `data-step`
does not have be continuous to pick the desired element.

For example:

```
<div id="first" data-step='5'></div>
<div id="second" data-step='9'></div>

// to get #second one can do now:
introJs().goToStepNumber(9); --> #second

// or with goToStep:
introJs().goToStep(2); --> #second
```

Author: doits

README.md

@@ -41,7 +41,6 @@ Optionally, pass one parameter to `introJs` function to limit the presentation s
 
 <p align="center"><img src="https://raw.githubusercontent.com/usablica/intro.js/gh-pages/img/introjs-demo.png"></p>
 
-
 ## Documentation
 
 Please visit [Documentation](http://introjs.com/docs).

docs/docs/intro/api.md

@@ -67,6 +67,36 @@ introJs().goToStep(2).start(); //starts introduction from step 2
 
 -----
 
+###introJs.goToStepNumber(step)
+
+Go to specific step of introduction with the concrete step.
+This differs from `goToStep` in the way that `data-step`
+does not have be continuous to pick the desired element.
+
+**Available since**: v2.x
+
+**Parameters:**
+
+ - step : Number
+
+**Returns:**
+
+ - introJs object.
+
+**Example:**
+
+```html
+<div id="first" data-step='5'></div>
+<div id="second" data-step='9'></div>
+````
+
+```javascript
+//start introduction from step with data-step='9'
+introJs().goToStepNumber(9).start();
+````
+
+-----
+
 ##### introJs.addStep(options)
 
 Add a new step to introJs.

intro.js

@@ -307,6 +307,19 @@
     }
   }
 
+  /**
+   * Go to the specific step of introduction with the explicit [data-step] number
+   *
+   * @api private
+   * @method _goToStepNumber
+   */
+  function _goToStepNumber(step) {
+    this._currentStepNumber = step;
+    if (typeof (this._introItems) !== 'undefined') {
+      _nextStep.call(this);
+    }
+  }
+
   /**
    * Go to next step on intro
    *
@@ -316,6 +329,16 @@
   function _nextStep() {
     this._direction = 'forward';
 
+    if (typeof (this._currentStepNumber) !== 'undefined') {
+        for( var i = 0, len = this._introItems.length; i < len; i++ ) {
+            var item = this._introItems[i];
+            if( item.step === this._currentStepNumber ) {
+                this._currentStep = i - 1;
+                this._currentStepNumber = undefined;
+            }
+        }
+    }
+
     if (typeof (this._currentStep) === 'undefined') {
       this._currentStep = 0;
     } else {
@@ -1721,6 +1744,11 @@
 
       return this;
     },
+    goToStepNumber: function(step) {
+      _goToStepNumber.call(this, step);
+
+      return this;
+    },
     nextStep: function() {
       _nextStep.call(this);
       return this;