Render to a stream

For demos, prototyping and SSR use-cases, it would be useful if `lit-html` supported rendering to a string. For example:

```ts
import {html, render} from 'lit-html';

const helloTemplate = (name) => html`<div>Hello ${name}!</div>`;

// This returns <div>Hello Steve!</div>
render(helloTemplate('Steve'));
```