[[_]] float clearing tag experiment

lib/gollum/frontend/public/gollum/css/gollum.css

@@ -729,3 +729,4 @@ ul.actions {
   padding: 0.25em;
 }
 
+.clearfloats { clear:both }

lib/gollum/markup.rb

@@ -236,6 +236,8 @@ module Gollum
     def process_tag(tag)
       if tag =~ /^_TOC_$/
         %{[[#{tag}]]}
+      elsif tag =~ /^_$/
+        %{<div class="clearfloats"></div>}
       elsif html = process_image_tag(tag)
         html
       elsif html = process_file_link_tag(tag)