Define extention plugins for Editor.md

[TOC]

### Define plugin

#### Plugin directory

editor.md/
        plugins/
            plugin-name/
                ....

#### Example

```javascript
(function() {

var factory = function (exports) {

var $ = jQuery;           // if using module loader(Require.js/Sea.js).

exports.customMethod = function() {
		    //....
		};

exports.fn.youPluginName = function() {
		    /*
			var _this       = this; // this == the current instance object of Editor.md
			var lang        = this.lang;
			var settings    = this.settings;
			var editor      = this.editor;
			var cursor      = cm.getCursor();
			var selection   = cm.getSelection();

cm.focus();
            */

//....
		};

};

// CommonJS/Node.js
	if (typeof require === "function" && typeof exports === "object" && typeof module === "object")
    {
        module.exports = factory;
    }
	else if (typeof define === "function")  // AMD/CMD/Sea.js
    {
		if (define.amd) { // for Require.js

define(["editormd"], function(editormd) {
                factory(editormd);
            });

} else { // for Sea.js
			define(function(require) {
                var editormd = require("./../../editormd");
                factory(editormd);
            });
		}
	} 
	else
	{
        factory(window.editormd);
	}

})();
```

#### Usage plugin

```html
<script src="../plugins/you-plugin-name/you-plugin-name.js"></script>
<script>
    editormd.customMethod();

var testEditor = editormd("test-editormd", {
        path   : '../lib/',
        onload : function() {
            this.youPluginName();
            this.pluginA();
            this.executePlugin("somePluginName", "you-plugin-name/you-plugin-name"); // load and execute plugin
        }
    });

// or
    $("#btn").click(function(){
        testEditor.youPluginName();
    });
</script>
```