add / createElements config option

I did a react component to wrap gemini-scrollbar, however, becauses
gemini-scrollbar inyects HTMLElements at runtime modifying the nodes
structure, when the react-component did update, the contents will be all
messed up. This is because gemini-scrollbar was infering with React
(Virtual) DOM.

To fix this, I added a new config-option `createElements` (default =>
true), this option tells gemini-scrollbar if it needs to create and inyect
the elements at runtime, or if it should search for them on the `element`
container instead. Passing this option as false, will assume that you to have
added the required markup with the specific css-classes on them to the lib to
work.

Example markup:
```
<div class="something-scrollable">

  <!-- required markup -->
  <div class="gm-scrollbar -vertical">
    <div class="thumb"></div>
  </div>
  <div class="gm-scrollbar -horizontal">
    <div class="thumb"></div>
  </div>
  <div class="gm-scroll-view">
    All your content here.
  </div>
  <!-- required markup -->

</div>

<script>
new GeminiScrollbar({
  element : document.querySelector('.something-scrollable'),
  createElements : false
}).create();
</script>
```

bower.json

@@ -1,7 +1,7 @@
 {
   "name": "gemini-scrollbar",
   "main": "index.js",
-  "version": "0.0.2",
+  "version": "0.0.3",
   "homepage": "https://github.com/noeldelgado/gemini-scrollbar",
   "authors": [
     "Noel Delgado <auhcsei@gmail.com>"

index.js

@@ -40,6 +40,7 @@
     function GeminiScrollbar(config) {
         this.element = null;
         this.autoshow = false;
+        this.createElements = true;
 
         Object.keys(config || {}).forEach(function (propertyName) {
             this[propertyName] = config[propertyName];
@@ -77,13 +78,22 @@
 
         this._document = document;
         this._window = window;
-        this._viewElement = document.createElement('div');
-        this._scrollbarVerticalElement = document.createElement('div');
-        this._thumbVerticalElement = document.createElement('div');
-        this._scrollbarHorizontalElement = document.createElement('div');
-        this._thumbHorizontalElement = document.createElement('div');
-        while(this.element.childNodes.length > 0) {
-            this._viewElement.appendChild(this.element.childNodes[0]);
+
+        if (this.createElements === true) {
+            this._viewElement = document.createElement('div');
+            this._scrollbarVerticalElement = document.createElement('div');
+            this._thumbVerticalElement = document.createElement('div');
+            this._scrollbarHorizontalElement = document.createElement('div');
+            this._thumbHorizontalElement = document.createElement('div');
+            while(this.element.childNodes.length > 0) {
+                this._viewElement.appendChild(this.element.childNodes[0]);
+            }
+        } else {
+            this._viewElement = this.element.querySelector('.' + CLASSNAMES.view);
+            this._scrollbarVerticalElement =this.element.querySelector('.' + CLASSNAMES.verticalScrollbar.split(' ').join('.'));
+            this._thumbVerticalElement = this._scrollbarVerticalElement.querySelector('.' + CLASSNAMES.thumb);
+            this._scrollbarHorizontalElement = this.element.querySelector('.' + CLASSNAMES.horizontalScrollbar.split(' ').join('.'));
+            this._thumbHorizontalElement = this._scrollbarHorizontalElement.querySelector('.' + CLASSNAMES.thumb);
         }
 
         this.element.classList.add(CLASSNAMES.element);

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gemini-scrollbar",
-  "version": "0.0.2",
+  "version": "0.0.3",
   "description": "Custom scrollbars with native scrolling",
   "license": "MIT",
   "repository": "noeldelgado/gemini-scrollbar.git",