Add a function for getting the post blank.

src/types/standard_properties.rs

@@ -9,6 +9,11 @@ pub trait StandardProperties<'s> {
     //
     // This corresponds to :contents-begin to :contents-end
     // fn get_contents(&'s self) -> &'s str;
+
+    // Get the slice of the AST node's post-blank text.
+    //
+    // This is optional whitespace following the node.
+    // fn get_post_blank<'b>(&'b self) -> &'s str;
 }
 
 // TODO: Write some debugging code to alert when any of the unknown fields below are non-nil in our test data so we can see what these fields represent.