Investigate using OOP and inheritance for meta tags

Meta tags share a lot of different properties, code, and UI. I think it might make sense to look into using an OOP architecture for meta tags. We would provide a base 'DrupalMetaTag' class which is then extended by the default meta tag classes (e.g. 'DrupalMetaTagDescription', 'DrupalMetaTagKeywords'). Other modules can provide alternate implementations. For example several other modules might want to generate the Description meta tag based on different methods. They both have a class that extends 'DrupalMetaTagDescription', and we expose a simple UI select field in the Description meta tag settings that gives you all the possible options: 'Default', 'Module A', 'Module B'.

Stub example code:

class DrupalMetaTag {
  abstract function getInfo() {
    //return array();
  }

  public function getForm() {
    // Return Drupal FAPI code here.
  }

  abstract function process($node) {
  }
}

class DrupalMetaTagDescription extends DrupalMetaTag {
  public function getInfo() {
    return array(
      'name' => 'Default',
      'provider' => 'meta_tags',
      'type' => 'description',
    );
  }

  public function process($node) {
    return truncate_utf8($node->body, 155);
  }
}

class ModuleAMetaTagDescription extends DrupalMetaTagDescription {
  public function getInfo() {
    return array(
      'type' => 'description',
      'provider' => 'module_a',
      'name' => 'Module A description method',
    );
  }

  public function process($node) {
    return module_a_summary($node);
  }
}

class ModuleBMetaTagDescription extends DrupalMetaTagDescription {
  public function getInfo() {
    return array(
      'type' => 'description',
      'provider' => 'module_B',
      'name' => 'Module B description method',
    );
  }

  public function process($node) {
    return module_b_summary($node);
  }
}