Added commandData to addWidgetCommand

Currently it's not possible to instantiate a new widget with existing data. 
See bug: http://dev.ckeditor.com/ticket/12770

>  According to the execCommand docs it should be possible to provide data to the execCommand ​http://docs.ckeditor.com/#!/api/CKEDITOR.editor-method-execCommand
> But when using this code none of that data ends up in the newly created widget, or in it's init or data methods
>     `editor.execCommand( 'simplebox', { align: 'right', opened: false } );`

This commit fixes the bug. 

Now new widgets can be instantiated with predefined data. 

    editor.execCommand( 'simplebox', { align: 'right', opened: false } );

Or completely custom

    var element = editor.createElement( 'div' );
    editor.insertElement( element );
    var widget = editor.widgets.initOn( element, 'simplebox', { align: 'right', opened: false } );

I've ran these tests with bender to confirm this does not break anything:

- name:nestedwidgets
- name:widgetapi
- name:widgets
- name:widgetselection
- name:widgetsintegration
- name:widgetsrepoapi
- path:/tests/plugins/widget
- tag:widget
- tag:widgetcore