Macro subgraph

macro_rules! subgraph {
    () => { ... };
    ($id:expr) => { ... };
    ($i:ident $id:expr) => { ... };
    ($id:expr, $stmts:expr) => { ... };
    ($i:ident $id:expr, $stmts:expr) => { ... };
    ($i:ident $id:expr; $($stmts:expr),+ ) => { ... };
    ($id:expr; $($stmts:expr),+ ) => { ... };
    (; $($stmts:expr),+ ) => { ... };
}

represents a subgraph in dot lang. #Example:

    fn subgraph_test() {
        use dot_generator::*;
        use dot_structures::*;

        assert_eq!(subgraph!(), Subgraph { id: Id::Anonymous("".to_string()), stmts: vec![] });
        assert_eq!(subgraph!("abc";node!()),
                   Subgraph {
                       id: Id::Plain("abc".to_string()),
                       stmts: vec![stmt!(node!())],
                   });
    }