def partial(filename)
      filename = partial_path(filename)
      raise "unable to find partial file: #{filename}" unless File.exists?(filename)
      contents = File.open(filename, "rb") {|f| f.read}
      # TODO: detect template EOL and match it to the partial's EOL
      # force unix eol
      contents.gsub!(/\r\n/, "\n") if contents.match("\r\n")
      contents
    end
def partial(filename)
      filename = partial_path(filename)
      raise "unable to find partial file: #{filename}" unless File.exists?(filename)
      contents = File.open(filename, "rb") {|f| f.read}
      # TODO: detect template EOL and match it to the partial's EOL
      # force unix eol
      contents.gsub!(/\r\n/, "\n") if contents.match("\r\n")
      contents
    end
render a partial

 filename: unless absolute, it will be relative to the main template

 @example slim escapes HTML, use '=='

   head
   == render 'mystyle.css'

 @return [String] of non-escaped textual content