Add webmaster_verifications for baidu (#263)

Merge pull request 263

docs/usage.md

@@ -54,6 +54,7 @@ webmaster_verifications:
   bing: 1234
   alexa: 1234
   yandex: 1234
+  baidu: 1234
 ```
 
 * `lang` - The locale these tags are marked up in. Of the format `language_TERRITORY`. Default is `en_US`.

lib/template.html

@@ -95,6 +95,10 @@
   {% if site.webmaster_verifications.yandex %}
     <meta name="yandex-verification" content="{{ site.webmaster_verifications.yandex }}" />
   {% endif %}
+
+  {% if site.webmaster_verifications.baidu %}
+    <meta name="baidu-site-verification" content="{{ site.webmaster_verifications.baidu }}" />
+  {% endif %}
 {% elsif site.google_site_verification %}
   <meta name="google-site-verification" content="{{ site.google_site_verification }}" />
 {% endif %}