Fix the links to the blog posts.

src/config/full.rs

@@ -57,6 +57,15 @@ impl Config {
         self.get_root_directory().join("posts")
     }
 
+    /// Get the relative path to the folder containing a blog post.
+    ///
+    /// This could be appended to the output root directory to get the
+    /// blog post output folder or it could be used to generate a link
+    /// to the blog post.
+    pub(crate) fn get_relative_path_to_post<P: AsRef<Path>>(&self, post_id: P) -> PathBuf {
+        Path::new("posts").join(post_id)
+    }
+
     pub(crate) fn get_output_directory(&self) -> PathBuf {
         self.get_root_directory().join("output")
     }