Outline potential dropdown configuration approach

This is from a discussion with Will about being able to configure dropdown options at block definition time.

This approach is to support a configuration string attribute on fields like this:

```
<title name="VALUE" config="50|100|200">100</title>
```

Where in the case of a `FieldDropdown`, the configuration constrains the available options for the block.
How about {Array.<!Number>}, to be precise