feat: add support for custom page titles

Also put the paste ID in the title for the show page

lib/ketbin_web/templates/layout/app.html.heex

@@ -4,7 +4,11 @@
     <meta charset="utf-8"/>
     <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
     <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
-    <title>Katbin</title>
+    <%= if Map.has_key?(assigns, :page_title) do %>
+      <title>Katbin - <%= @page_title %></title>
+    <% else %>
+      <title>Katbin</title>
+    <% end %>
     <link rel="stylesheet" href={ Routes.static_path(@conn, "/assets/app.css") }/>
     <script defer type="text/javascript" src={ Routes.static_path(@conn, "/assets/app.js") }></script>
   </head>