Support or document non-generated accessors.

[justinfagnani]

We should make sure that handwritten accessors are fully supported, documented, and tested.

I assume this much works to delegate state to Polymer, but does it properly call render()?

class MyElement extends LitElement {
  get foo() { return this._getProperty('foo'); }
  set foo(v) { this._setProperty('foo'); }

  render() {
    return html`foo: ${this.foo}`;
  }
}

[sorvell]

This is supported and tested.

[nonmetalhail]

It would be lovely if you could add this to the documentation :)