User can select fields for form in shortcode

This plugin was what my client was looking for, but they didn't want all the fields in the front-end form for user submissions. I added a *fields* parameter to the shortcode that allows users to pass in optional fields they want to add. By default __title__, __description__, and __name__ will display- because what's a good testimonial without those :smile: 

CAPTCHA and the hidden fields are outside the conditional for the args, so they display no matter what, as well.