Add two new methods, getBlocksByType and getBlocksByIDs to the editor.
Merge branch 'v3' of github.com:madebymany/sir-trevor-js into v3

* 'v3' of github.com:madebymany/sir-trevor-js:
  Extract Pastable & Uploadable into separate mixins. Has a few BREAKING API changes by doing this, had to change the image, tweet and video blocks to work with the new format. Also, have added a new  element when the block is pasteable / uploadable or droppable. This allows us to easily show / hide controls as before without tying ecverything to droppable.