add simple plugin system

I add a simple plug-in feature to the Summernote.

```javascript
// define plugin 
$.summernote.plugins = {

"chart" : { 
    label : '',  // default button label
    icon : '',    // if it has a icon property, there are create a icon button. 
    dropdown : function () {  // if you want to make a dropdown button, use dropwdown property.
        return "<div class='dropdown-menu'>...blabla </div>";
    },
    event :  function (e, editor, layout) {   // callback  function 
    },
}

}

// use plugin 
$(".xxx").summernote({
    toolbar : [
    ......,
       ['mychart' , ['chart'] ]   // write plugin name 
    ]
});

```
Reference to examples/plugin.html for detail plugin code.