Improvements to mask drawing utility

[afluerasu]

The mask drawing utility needs to be able to handle drawing straight lines between two points. It does not appear to handle this right now.

[tacaswell]

@afluerasu You just need a very steady hand.

[afluerasu]

@tacaswell , It's difficult..
You see? :)

[tacaswell]

The tooling to do this exist, we just need to sort out how to integrate them. See https://github.com/scikit-image/scikit-image/blob/master/viewer_examples/plugins/lineprofile.py for example.