Adds 'with' option to schema for specifying the Field object constructor to build the field with

Sometimes certain fields in a form require custom logic. This allows you to use different Field constructors for different fields in the schema using a `with` option like so:

```javascript
schema: {
  name: {
    title: "Full Name",
    with: MyCoolNameField
  }
}
```

Thoughts? If this is something we'd like to merge, I'd be happy to add tests.