Method inside_header

```rust use orgize::{Org, ast::InlineCall}; let call = Org::parse("call_square[:results output](4)").first_node:: ().unwrap(); assert_eq!(call.inside_header().unwrap(), ":results output"); let call = Org::parse("call_square(4)[:results html]").first_node:: ().unwrap(); assert!(call.inside_header().is_none()); ```

(&self)