Add support for selecting multiple editable blocks

[anthonyjb]

Please see original issue here: GetmeUK/ContentTools#569

So the idea of being able to select multiple paragraphs in order to convert all of them to a list is something I really like and it would be nice also to be able to perform a move, delete or merge against multiple blocks of content. All these actions should be possible though this still wont provide support for selecting text across multiple paragraphs (this is a more difficult problem and one I'm not sure at this point we'll look to resolve in CT 2).