Implement the export snippet parser.

src/parser/object.rs

@@ -114,7 +114,7 @@ pub struct LatexFragment<'s> {
 pub struct ExportSnippet<'s> {
     pub source: &'s str,
     pub backend: &'s str,
-    pub contents: &'s str,
+    pub contents: Option<&'s str>,
 }
 
 impl<'s> Source<'s> for Object<'s> {