Create example of plugin for napari viewer

Anticipating the [Alfa Cohort Showcase](https://chanzuckerberg.github.io/napari-plugin-alfa-cohort/) that will take place on `Thursday, April 29th, 2021 from 8:00AM-1:00PM, Pacific Time (PT)`, it could make sense to develop a skeleton of ITK based plugin for napari viewer that would enable the use of ITK python.

> Focusing on detection and segmentation of cells, nuclei, and structures, we seek to (1) establish some of the first analysis plugins for the open source napari, (2) ensure that the platform meets the needs of these tools and their users, and (3) work together to make napari the best place to build and share image analysis methods.

Relevant references:
* https://napari.org/docs/dev/plugins/index.html