Custom copy support

[Reinmar]

Right now (after ckeditor/ckeditor5-clipboard#1) we support pasting in a custom way. This means that we intercept the content and insert it as we wish (e.g. filter stuff out and don't allow to break our custom data model :)).

Custom copy support is needed for two reasons:

• On Webkit the HTML format is not yet available in the clipboard unless pushed there by us (on copy). This issue is already fixed by the Webkit team but it will certainly take some time before the patch lands in stable browsers.
• We need to be able to put arbitrary HTML in the clipboard when there's a fake selection, or in other words – when a widget is selected.