Method key

```rust use orgize::{Org, ast::Macros}; let m = Org::parse("{{{title}}}").first_node:: ().unwrap(); assert_eq!(m.key(), "title"); let m = Org::parse("{{{two_arg_macro(1, 2)}}}").first_node:: ().unwrap(); assert_eq!(m.key(), "two_arg_macro"); ```

(&self)