kebab-case event handlers

It's more common to see custom events with kebab-case event type strings (e.g. [old polymer](https://github.com/Polymer/polymer/blob/17720d01883358c4bb6ac02fc9e51a6cc2336e44/lib/mixins/property-effects.js#L280)). We need a way to customize the wrapper to account for that.

Or it's already possible?