Config option to hide image2's resizer

[Piotrek Koszuliński]

As explained in ​http://dev.ckeditor.com/ticket/12615#comment:14 and ​https://github.com/ckeditor/ckeditor-docs/issues/51 we need an option to disable the resizer without removing the width and height attribute (it's now possible to disallow them and resizer automatically disappears).

I'd also love if we could extract the code needed for the resizer to a separate plugin. The setupResizerfunction has 200LOC plus there's 30LOC of CSS, so that could be something. And there's nothing more there, so that would we an entire feature, which after some improvements perhaps could be reused by other plugins (like video embedding). But I'm afraid that we won't have enough time...