Support the university_category_slug attribute in wsuwp_json shortcode

If a slug is supplied, any other manual query attribute specified will
be ignored
Fire an action when wsuwp_json is used to embed JSON data
esc_url() eats square brackets, only escape the first part of the URL
Allow themes or plugins to filter the HTML output by the shortcode
Merge pull request #6 from washingtonstateuniversity/university-category

[PR] Support a university_category_slug attribute in the wsuwp_json shortcode