Pass datepicker arguments through a filter and fix error with empty dateFormat

This PR completes PR #383 and correctly stages it against the development branch.

@kstover, I found three places in the code where the datepicker arguments were passed to the JavaScript. These are:

* ```classes/subs-cpt.php -> assets/js/dev/subs-cpt.js```
* ```includes/admin/scripts.php -> js/dev/ninja-forms-admin.js```
* ```includes/display/scripts.php -> js/dev/ninja-forms-display.js```

I've created a ```$datepicker_args``` array to pass to each JavaScript file and passed each array through a uniquely named filter. However, I was unable to find any field that used the datepicker. Is this only available in an addon?

For that reason, I was only able to test the datepicker fix on the Forms > Submissions admin page (which loads the ```assets/js/dev/subs-cpt.js file```). I did check via the console that the datepicker_args object is getting correctly passed (with no dateFormat argument) to the other two JavaScript files, though.