Add target attribute for navigation link (#3056)

_includes/masthead.html

@@ -14,7 +14,11 @@
         <ul class="visible-links">
           {%- for link in site.data.navigation.main -%}
             <li class="masthead__menu-item">
-              <a href="{{ link.url | relative_url }}"{% if link.description %} title="{{ link.description }}"{% endif %}>{{ link.title }}</a>
+              <a
+                href="{{ link.url | relative_url }}"
+                {% if link.description %} title="{{ link.description }}"{% endif %}
+                {% if link.target %} target="{{ link.target }}"{% endif %}
+              >{{ link.title }}</a>
             </li>
           {%- endfor -%}
         </ul>