Generate Post link when the protocol is post.

src/intermediate/regular_link.rs

@@ -41,14 +41,24 @@ intermediate!(
 #[derive(Debug, Clone)]
 pub(crate) enum LinkTarget {
     Raw(String),
-    Post { post_id: String },
+    Post { post_id: Option<String> },
 }
 
 impl LinkTarget {
     pub(crate) fn from_string<S: AsRef<str>>(input: S) -> Result<LinkTarget, CustomError> {
         fn impl_from_string(input: &str) -> Result<LinkTarget, CustomError> {
             let parsed = Url::parse(input)?;
-            Ok(LinkTarget::Raw(input.to_owned()))
+            match parsed.scheme() {
+                "post" => {
+                    let post_id = parsed
+                        .path_segments()
+                        .ok_or_else(|| "cannot be base")?
+                        .next()
+                        .map(str::to_owned);
+                    Ok(LinkTarget::Post { post_id })
+                }
+                _ => Ok(LinkTarget::Raw(input.to_owned())),
+            }
         }
         impl_from_string(input.as_ref())
     }