Use LD+JSON instead of attributes in the markup

[celsobessa]

This is an early issue to spark discussion and track my contributions.

• LD+JSON is the preferred structured data markup for Google and Bing is improving it's support to it.
• Also, schema.org markup using attributes in the content HTML markup is less flexible for developers who want to change the elements used in the testimonial

For these reasons, the plugin should use LD+JSON for the structured data