Hello. I created a webpack based starter kit. It is available at: https://github.com/mfoitzik/prosemirror-breakout-starter-kit
It took me awhile for the concepts of ProseMirror to click (I still have a lot of work to do to get more things to click with me ), especially in regards to adding functionality. This starter kit is basically the basic example but breaks out the pieces of the ProseMirror-example-setup package.
The README goes through the steps required to add functionality (I added strikethrough just to keep things simple). It adds the entry in the schema, the keyboard shortcut to support it and adds a new button to the menu.
I am hoping it may help some of you out.