Allow users to register element names

https://github.com/esxjs/esx looks a lot like HTM, but instead of handling components as `<${Foo}>`, you have to (get to?) pre-register component names, like this: `esx.register({Foo})`

Then you can use custom components like this:

```js
esx`Blah`
```

It would be cool if this were an option for HTM as well.