[feat] search: shift/ctrl click a category to select multiple categories

searx/templates/simple/categories.html

@@ -31,6 +31,7 @@
                     <div class="category_name">{{- _(category) -}}</div>{{- '' -}}
                 </button>{{- '' -}}
             {%- endfor -%}
+            <input name="categories" id="selected-categories" type="hidden" />
             {{- '\n' -}}
         {%- endif -%}
     </div>{{- '' -}}