Support blank lines before content in footnote definitions.

org_mode_samples/greater_element/footnote_definition/empty_space_before_and_after_content.org

@@ -6,4 +6,3 @@
 baz
 #+END_EXAMPLE
 
-

src/parser/footnote_definition.rs

@@ -4,8 +4,10 @@ use nom::bytes::complete::tag_no_case;
 use nom::bytes::complete::take_while;
 use nom::character::complete::digit1;
 use nom::character::complete::space0;
+use nom::combinator::opt;
 use nom::combinator::recognize;
 use nom::combinator::verify;
+use nom::multi::many0;
 use nom::multi::many1;
 use nom::multi::many_till;
 use nom::sequence::tuple;
@@ -41,8 +43,15 @@ pub fn footnote_definition<'b, 'g, 'r, 's>(
     }
     start_of_line(input)?;
     // Cannot be indented.
-    let (remaining, (_lead_in, lbl, _lead_out, _ws)) =
-        tuple((tag_no_case("[fn:"), label, tag("]"), space0))(input)?;
+    let (remaining, (_, lbl, _, _, _)) = tuple((
+        tag_no_case("[fn:"),
+        label,
+        tag("]"),
+        space0,
+        opt(verify(many0(blank_line), |lines: &Vec<OrgSource<'_>>| {
+            lines.len() <= 2
+        })),
+    ))(input)?;
     let contexts = [
         ContextElement::ConsumeTrailingWhitespace(true),
         ContextElement::Context("footnote definition"),