Feature: JSON+LD schema markup

[acobster]

As a Conifer dev user, I want to be able to output schema markup for posts as JSON-LD data. Something like:

<article>
  <h2>{{ post.title }}</h2>
  <script type="application/ld+json">{{ post.json_ld() }}</script>
</article>

Acceptance Criteria

• [ ] Conifer should include a trait, Conifer\Schema\HasStructuredDataMarkup, that requires the including class to implement a ::schema_markup() method which returns an array. The trait itself should implement ::json_ld() which calls ::schema_markup() and returns the JSON-serialized schema data.
• [ ] Implement and document the minimum set of schemas to support, e.g. Article.
• [ ] Document how to extend implement your own schemas, e.g. Event.

Possible usage:

namespace Project\Post;

use Conifer\Post\Post;
use Conifer\Schema\HasStructureDataMarkup;

class Event extends Post {
  use HasStructuredDataMarkup;

  const POST_TYPE = 'event';

  protected function schema_markup() : array {
    return [
      '@type'    => 'Event',
      'headline' => $this->title(),
      // ...
    ];
  }
}

In the future, we may look at enhancing the trait with other microformats, although there probably isn't a big advantage there.