Add variable to optionally override base portion of canonical url

Hey @ryanissamson!

A coupla things:

1. Hello again!
2. I just invited you to join the Drupal Instant Articles development FB group - check Facebook :)
3. Can you add this as a GitHub PR rather than a patch, and we can discuss there? Check out the "Issues and Development" section of the project landing page:
   

   All development is happening via PRs in GitHub

4. When you do, please make the PR against 7.x-2.x rather than 7.x-1.x (we're now adding all new features to the 7.x-2.x branch… a release tag from that branch should be coming soon).
5. Regarding this issue, interestingly I had just done this in a hook rather than building in a config. I'm happy to share that code, but hearing that others besides us may need this, a config sounds like a good new feature.

Hey :) OK, here is essentially what we had just done:

For 7.x-1.x

hook_fb_instant_articles_display_instant_article_alter(\Facebook\InstantArticles\Elements\InstantArticle $instant_article, $node) {
  if ($canonical_domain = variable_get('fb_instant_articles_canonical_domain')) {
    global $base_path;
    $url = $canonical_domain . $base_path . drupal_get_path_alias('node/' . $node->nid);
    $instant_article->withCanonicalURL($url);
  }
}

For 7.x-2.x

function hook_fb_instant_articles_article_alter($instantArticle, $context) {
  if (isset($context['entity_type']) && isset($context['entity']) && $canonical_domain = variable_get('fb_instant_articles_canonical_domain')) {
    global $base_path;
    $path = entity_uri($context['entity_type'], $context['entity']);
    $url = $canonical_domain . $base_path . drupal_get_path_alias($path);
    $instant_article->withCanonicalURL($url);
  }
}

Merged! Nice feature