Method get

```rust use orgize::{Org, ast::PropertyDrawer}; let org = Org::parse("* Heading\n:PROPERTIES:\n:CUSTOM_ID: someid\n:ID: id\n:END:"); let drawer = org.first_node:: ().unwrap(); assert_eq!(drawer.get("CUSTOM_ID").unwrap(), "someid"); assert_eq!(drawer.get("ID").unwrap(), "id"); ```

(&self, key: &str)