Implement Facebook domain verification

https://developers.facebook.com/docs/sharing/domain-verification/

docs/usage.md

@@ -65,6 +65,7 @@ The following properties are available:
     alexa: 1234
     yandex: 1234
     baidu: 1234
+    facebook: 1234
   ```
 * `locale` - The locale these tags are marked up in. Of the format `language_TERRITORY`. Default is `en_US`. Takes priority
 over existing config key `lang`.

lib/template.html

@@ -107,6 +107,10 @@
   {% if site.webmaster_verifications.baidu %}
     <meta name="baidu-site-verification" content="{{ site.webmaster_verifications.baidu }}" />
   {% endif %}
+
+  {% if site.webmaster_verifications.facebook %}
+    <meta name="facebook-domain-verification" content="{{ site.webmaster_verifications.facebook }}" />
+  {% endif %}
 {% elsif site.google_site_verification %}
   <meta name="google-site-verification" content="{{ site.google_site_verification }}" />
 {% endif %}

spec/jekyll_seo_tag_integration_spec.rb

@@ -618,6 +618,7 @@ RSpec.describe Jekyll::SeoTag do
           "bing"   => "bar",
           "alexa"  => "baz",
           "yandex" => "bat",
+          "facebook" => "bas",
         }
       end
 
@@ -642,6 +643,11 @@ RSpec.describe Jekyll::SeoTag do
         expected = %r!<meta name="yandex-verification" content="bat" />!
         expect(output).to match(expected)
       end
+
+      it "outputs facebook verification meta" do
+        expected = %r!<meta name="facebook-domain-verification" content="bas" />!
+        expect(output).to match(expected)
+      end
     end
 
     context "with site.google_site_verification" do