message: Add GraphContentOnly config for dot
diff: @@ -80,6 +80,8 @@ pub enum Config {
  EdgeIndexLabel,
  /// Use no edge labels.
  EdgeNoLabel,
  + /// Do not print the graph/digraph string.
  + GraphContentOnly,
  #[doc(hidden)]
  _Incomplete(()),
  }
  @@ -97,7 +99,9 @@ impl<'a, G> Dot<'a, G>
  NF: FnMut(&NW, &mut FnMut(&Display) -> fmt::Result) -> fmt::Result,
  EF: FnMut(&EW, &mut FnMut(&Display) -> fmt::Result) -> fmt::Result,
  {
  + if !self.config.contains(&Config::GraphContentOnly) {
  try!(writeln!(f, "{} {{", TYPE[g.is_directed() as usize]));
  + }
  // output all labels
  for node in g.node_references() {
  @@ -129,7 +133,9 @@ impl<'a, G> Dot<'a, G>
  }
  }
  + if !self.config.contains(&Config::GraphContentOnly) {
  try!(writeln!(f, "}}"));
  + }
  Ok(())
  }
  }
  