add search to the navbar using 'post_search' config param

_includes/nav.html

@@ -30,6 +30,14 @@
           </li>
         {%- endif -%}
       {%- endfor -%}
+      {% if site.post_search %}
+        <li class="nav-item">
+          <a class="nav-link" id="nav-search-link" href="#" title="Search">
+            <span id="nav-search-icon" class="fa fa-search"></span>
+            <span id="nav-search-text">Search</span>
+          </a>
+        </li>
+      {%- endif -%}
     </ul>
   </div>
 
@@ -50,3 +58,5 @@
   {% endif %}
 
 </nav>
+
+{% include search.html %}