[Form] Array to delimited string transformer

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | [not yet]

This data transformer transforms an array of values to a delimited string.

e.g.

````
array('foo', 'bar') => 'foo, bar'
````