Allow to disable chapter anchors

lib/repubmark/elems/chapter.rb

@@ -67,8 +67,7 @@ module Repubmark
       def anchor
         if parent.instance_of?(self.class) && @level > 2
           "#{parent.anchor}-#@slug"
-        elsif (anchor_prefix =
+        elsif anchor_prefix.empty?
-               String(config[:chapter_anchor_prefix]).strip).empty?
           @slug
         else
           "#{anchor_prefix}-#@slug"
@@ -133,12 +132,17 @@ module Repubmark
       end
 
       def build_title_html
-        %(<h#@level id="#{anchor}">#@title</h#@level>\n) if @level != 1
+        id_attr = %( id="#{anchor}") unless anchor_prefix.empty?
+        %(<h#@level#{id_attr}>#@title</h#@level>\n) if @level != 1
       end
 
       def build_title_gemtext
         "\n#{'#' * @level} #@title\n\n" if @level != 1
       end
+
+      def anchor_prefix
+        @anchor_prefix ||= String(config[:chapter_anchor_prefix]).strip.freeze
+      end
     end
   end
 end