Method is_archived

Return `true` if this headline contains an archive tag ```rust use orgize::{Org, ast::Headline}; let hdl = Org::parse("* hello :ARCHIVE:").first_node:: ().unwrap(); assert!(hdl.is_archived()); let hdl = Org::parse("* hello :ARCHIVED:").first_node:: ().unwrap(); assert!(!hdl.is_archived()); ```

(&self)