Add support for custom formats in examples

This adds the ability to add custom `formats` in the examples by simply
adding an `meta.yaml` file to the example with something like this:

```yaml
formats:
  date:
    timeStyle:
      hour: numeric
      minute: numeric

  number:
    percentStyle:
      style: percent
```

This also improves the "Render" tab by actually outputting the
`formats` and `messages` used by the example.
Merge pull request #114 from ericf/examples-with-formats

Add support for custom formats in examples