Add an option to rearrange all the buttons and group buttons

This PR will fix this issue: https://github.com/toopay/bootstrap-markdown/issues/41

It give the possibility to:
* Rearrange all buttons & group buttons in whatever order;
* Remove any button that we don't need;
* And of course move the preview button after any additional buttons.

The new option `rearrangeButtons` take an array of values.
In this array we could add:
* A `string`: corresponding to an existing group-name;
* An `object`: containing a name (the object property) + an array of existing button-name (the object value).

Here a complete sample code:

```js
$("#target-editor-with-custom-buttons").markdown({
  additionalButtons: [
    [{
      name: "groupCustom",
      data: [{
        name: "cmdBeer",
        toggle: true,
        title: "Beer",
        icon: "glyphicon glyphicon-glass",
        callback: function(e){}
      },
      {
        name: 'cmdSave',
        toggle: false,
        title: 'Save',
        icon: 'glyphicon glyphicon-floppy-disk',
        callback: function () {}
      },
      {
        name: 'cmdFullscreen',
        toggle: false,
        title: 'Fullscreen',
        icon: 'glyphicon glyphicon-fullscreen',
        callback: function () {}
      }]
    }]
  ],
  rearrangeButtons: [
    {
      'groupBeer': [
        'cmdBeer'
      ]
    },
    'groupFont',
    'groupLink',
    {
      'groupMisc': [
        'cmdList',
        'cmdSave'
      ]
    },
    {
      'groupUtil': [
        'cmdFullscreen',
        'cmdPreview'
      ]
    }
  ]
})
```

Probably this weekend, I will send another PR to update the documentation on the website.