array field output handler

Adds a new field output handler to return multi-value fields as an array.

My use case: I have an API that sends event data, but I have a multi-value custom field ("Category") which I used with the "OptionFieldOutputHandler". This would return comma-separated values if multiple categories were selected, but I needed them to be a JSON array.

This handler adds the ability to return arrays of multi-value fields (thanks for the isMultiValue property - seems unused elsewhere but very helpful to me!). You can specify in the configuration if you want to return option values or labels.