…dow DOM

The current Web Components compiler produces an invalid Custom Element. When instantiated via `document.createElement('icon-a-b')`, it will throw an error: "Failed to construct 'CustomElement': The result must not have children".  The recommended solution from the CE spec is to append light DOM children in `connectedCallback`, so I've made that change here. I've also added a `shadow` option to have the generated CE use an open Shadow Root instead of light DOM children.