Add an "example" property to the field config schema

Adding the `eg` property to fields, allowing implementors to provide example values to end-users to improve their experience.  These can be used as placeholder text or in validation messages.

For instance, here's the definition of the US zip code field, with an example value suitable for use as placeholder text:
```json
{
  "label": "Zip code",
  "format": "^\\d{5}(?:[-\\s]\\d{4})?$",
  "eg": "98103"
}
```