I've been exploring KonvaJs for a while. And I like it. I've done some pretty awesome animations using this library too.
The more you use the library, the more you'd want to make a custom shape. And there is a solution for that. However, that talks about manually drawing shapes using the 2d canvas primitives. But, you'd also want to re-use what is already offered by the library. Konva docs currently don't have a say about how to do this.
I may have found a hack. It may not be elegant when you think it. But it will do...
Here is my git repo if you are interested in code for an example.
Ps: you'll need npm and gulp to run it.