Parse medium URLs

As far as I can tell, the post id for all medium posts is always 12 hex
characters. We'll find out if that's true.

src/actions/articles/show.cr

@@ -1,7 +1,17 @@
 require "json"
 
 class Articles::Show < BrowserAction
-  get "/posts/:post_id" do
+  get "/posts/:post_slug" do
+    id_match = post_slug.match(/([0-9a-f]{12})$/i)
+    if id_match
+      post_id = id_match[1]
+    else
+      return html(
+        Errors::ShowPage,
+        message: "Error parsing the URL",
+        status: 500,
+      )
+    end
     if Lucky::Env.use_local?
       response = LocalClient.post_data(post_id)
     else