Added ckeditor plugin registry

[wachterjohannes]

This PR adds the ability to register ckeditor plugins.

Example:

In a aura-extension:

sandbox.ckeditor.addPlugin(
    'internalLink',
    {
        init: function(editor) {
...
        }
    }
);
sandbox.ckeditor.addToolbarButton('links', 'InternalLink');

[danrot]

Why do we need 'ckeditor' and 'jqueryAdapter' here, although it is not passed to the function?

[wachterjohannes]

we need it to be sure that it is loaded by require. this scripts will add some global vars and constructor's we need.

[wachterjohannes]

@danrot fixed