ability to intercept Base64 dataURL images before upload (server overload)

[jywarren]

We've seen some dataURL images pasted into posts and that can badly overload our database when it's saved as a string into a text record instead of uploaded separately.

Not sure if this will require code changes, but i'd like to intercept this if possible and upload via as normal images are!

[jywarren]

This could be doable in a similar fashion to: #38

[jywarren]

At minimum a way to insert a filter to find these so we can do /something/ like show an alert, or prevent upload, or replace them with something.