Implement hidden article

Signed-off-by: Minecon724 <git@m724.eu>

src/main/java/eu/m724/blog/BlogBuilder.java

@@ -205,6 +205,9 @@ public class BlogBuilder {
                 } catch (FileAlreadyExistsException ignored) { }
 
                 Files.writeString(outFile, render);
+
+                // TODO Is this the right way? To just ignore hidden? Right now it doesn't matter, but in the future?
+                if (!article.hidden())
                     articles.add(article);
             }
         }

src/main/java/eu/m724/blog/template/TemplateExtension.java

@@ -61,10 +61,28 @@ public class TemplateExtension extends AbstractExtension {
                             path = CacheBuster.insertHashInPath(path, hash);
                         }
 
+                        return site.directory() + path;
+                    }
+                },
+                "url_for", new Function() {
+                    @Override
+                    public List<String> getArgumentNames() {
+                        return List.of("path");
+                    }
+
+                    @Override
+                    public Object execute(Map<String, Object> args, PebbleTemplate self, EvaluationContext context, int lineNumber) {
+                        var path = args.get("path").toString();
+
+                        if (path.startsWith("/")) {
+                            path = path.substring(1);
+                        } else {
+                            // TODO Should we apply special treatment? Relative path?
+                        }
+
                         return site.directory() + path;
                     }
                 }
-                // TODO make url_for that supports relative and absolute paths
         );
     }