Add trailing_slash option for prettier urls

"set :trailing_slash, false" will cause resource urls that match the
index_file to have the trailing slash stripped off the directory URL,
e.g. instead of "/dir/index.html" becoming "/dir/" it will be "/dir"

middleman-core/lib/middleman-core/application.rb

@@ -93,6 +93,10 @@ module Middleman
     # @return [String]
     set :index_file,  "index.html"
 
+    # Whether to include a trailing slash on links to directory indexes
+    # @return [Boolean]
+    set :trailing_slash, true
+
     # Location of javascripts within source.
     # @return [String]
     set :js_dir,      "javascripts"

middleman-core/lib/middleman-core/sitemap/resource.rb

@@ -142,7 +142,7 @@ module Middleman
       # @return [String]
       def url
         File.join(app.respond_to?(:http_prefix) ? app.http_prefix : '/',
-          destination_path.sub(/\/#{Regexp.escape(app.index_file)}$/, '/'))
+          destination_path.sub(/\/#{Regexp.escape(app.index_file)}$/, app.trailing_slash ? '/' : ''))
       end
     end
   end