Get post blank for footnote references.

src/types/object.rs

@@ -193,6 +193,7 @@ pub struct ExportSnippet<'s> {
 pub struct FootnoteReference<'s> {
     pub source: &'s str,
     pub contents: Option<&'s str>,
+    pub post_blank: Option<&'s str>,
     pub label: Option<&'s str>,
     pub definition: Vec<Object<'s>>,
 }
@@ -736,7 +737,11 @@ impl<'s> StandardProperties<'s> for FootnoteReference<'s> {
     }
 
     fn get_post_blank<'b>(&'b self) -> PostBlank {
-        todo!()
+        self.post_blank
+            .map(|text| text.chars().count())
+            .unwrap_or(0)
+            .try_into()
+            .expect("Too much post-blank to fit into a PostBlank.")
     }
 }