Update tutorial for adding new/custom extensions

[duckontheweb]

The adding-new-and-custom-extensions.ipynb tutorial is not up-to-date with the new way of handling extensions since #309. We should update this tutorial to reflect the correct way of contributing support for a new extension and creating custom extensions.

Originally raised in #376

[duckontheweb]

Pushing this to the next release so we don't hold up 1.3.0