module Middleman::CoreExtensions::DefaultHelpers::Helpers

def link_to(*args, &block)

to config.rb to have all links default to relative.

set :relative_links, true

relative URLs instead of absolute URLs. You can also add
:relative option which, if set to true, will produce
you'll get that resource's nice URL. Also, there is a
reference a source path, either absolutely or relatively,
Overload the regular link_to to be sitemap-aware - if you 
def link_to(*args, &block)
  url_arg_index = block_given? ? 0 : 1
  options_index = block_given? ? 1 : 2
  if url = args[url_arg_index]
    options = args[options_index] || {}
    relative = options.delete(:relative)
    
    if url.include? '://'
      raise "Can't use the relative option with an external URL" if relative
    else
      # Handle relative urls
      current_dir = Pathname('/' + current_resource.path).dirname
      path = Pathname(url)
      url = current_dir.join(path).to_s if path.relative?
      resource = sitemap.find_resource_by_path(url)
      
      # Allow people to turn on relative paths for all links with set :relative_links, true
      # but still override on a case by case basis with the :relative parameter.
      effective_relative = relative || false
      if relative.nil? && relative_links
        effective_relative = true
      end
      if resource
        if effective_relative
          resource_url = resource.url
          new_url = Pathname(resource_url).relative_path_from(current_dir).to_s
          # Put back the trailing slash to avoid unnecessary Apache redirects
          if resource_url.end_with?('/') && !new_url.end_with?('/')
            new_url << '/'
          end
        else
          new_url = resource.url
        end
        args[url_arg_index] = new_url
      else
        raise "No resource exists at #{url}" if relative
      end
    end
  end
  super(*args, &block)
end