Add support for planning before property drawer when calculating additional properties for headlines.

src/types/document.rs

@@ -101,11 +101,15 @@ impl<'s> Heading<'s> {
                 _ => None,
             })
             .flat_map(|section| section.children.iter())
-            .take(1)
-            .filter_map(|element| match element {
+            .take_while(|element| match element {
+                Element::Planning(_) | Element::PropertyDrawer(_) => true,
+                _ => false,
+            })
+            .find_map(|element| match element {
                 Element::PropertyDrawer(property_drawer) => Some(property_drawer),
                 _ => None,
             })
+            .into_iter()
             .flat_map(|property_drawer| property_drawer.children.iter())
     }
 }