Allow multiple options to be passed via custom_args_js_event

Currently, you can only pass one value through your `custom_args_js_event` event without some creative coding.

The current [event trigger](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/d739dabaebeb40f9a4fc551735ace627a6979982/js/fieldmanager-autocomplete.js#L32) could be modified to pass an object as an argument. This object will be passed by reference, and any hook will be able to modify it and set e.g. `args.category = 123;`.

The current implementation is:

``` js
// Check for custom args
var custom_args_js_event = $el.data( 'customArgsJsEvent' );
var custom_data = '';
if ( 'undefined' !== typeof custom_args_js_event && null !== custom_args_js_event ) {
var custom_result = $el.triggerHandler( custom_args_js_event );
if ( 'undefined' !== typeof custom_result && null !== custom_result ) {
custom_data = custom_result;
}
}
```

A better implementation would be:

``` js
// Check for custom args
var custom_args_js_event = $el.data( 'customArgsJsEvent' );
var custom_data = {};
if ( 'undefined' !== typeof custom_args_js_event && null !== custom_args_js_event ) {
$el.triggerHandler( custom_args_js_event, custom_data );
}
```

We need to be concerned with backwards compatibility here, and maybe check the result and if the result is undefined, then look to the object.