Adapt for TW 3.0.0RC1

* Don't show donation or premium upgrade links by default
* Removed filter `twp_next_text`
* RESOLVE michael-cannon/testimonials-widget#103 Example of every option
* RESOLVE michael-cannon/testimonials-widget#107 Make text labels easier to replace
* RESOLVE michael-cannon/testimonials-widget#109 Open full testimonial in a lightbox #109
* RESOLVE michael-cannon/testimonials-widget#123 Revise installation and usage instructions for ease of use
* RESOLVE michael-cannon/testimonials-widget#19 Template engine