Fix complex field types: input[type="checkbox"] and select[multiple]

[kaidjohnson]

input[type="checkbox"] can represent boolean, string, and string[] data. select[multiple] expects an array, but event.target.value only represents a single string value.

Vue has resolved this logic within v-model (https://vuejs.org/v2/guide/forms.html#Checkbox, https://vuejs.org/v2/guide/forms.html#Checkbox-1). This update adds a change event listener to handle these more complex use-cases, allowing access to the value bound via v-model and updating the final-form state accordingly.

Addresses #5