Added option updateOnBlurOnly to update ngmodel only on editor blur

[prp1]

Updating ngmodel can be an expensive operation, and sometimes it is sufficient to update it when editing is done - on blur event.

My particular case was that if there are many editors in the page, in IE11 when writing something in the editor a big lag is visible.

This pull request allows to fix this problem.

[deeg]

Thanks, sorry for the late response.

I would rather see us properly implement ng-model-options, but I know this is a big feature request.

I'm okay with doing this in the interim. I will try and review and get merged in later today.