restructured abi of main class

src/Parkdown.php

@@ -2,17 +2,26 @@
 
 namespace parkdown;
 
+use DOMDocument;
+
 class Parkdown {
-	private string $sourceCode;
+	private string      $sourceCode;
+	private DOMDocument $tree_;
 
 	public function __construct(string $sourceCode) {
-		$this->sourceCode= $sourceCode;
-	}
+		$this->sourceCode = $sourceCode;
 
-	public function html() : void {
 		$lexer  = new Lexer($this->sourceCode);
 		$parser = new Parser($lexer->tokenize());
+	
+		$this->tree_ = $parser->parse();
+	}
+
+	public function tree() : DOMDocument {
+		return $this->tree_;
+	}
 
-		echo $parser->parse();
+	public function html() : string {
+		return $this->tree_->saveHTML();
 	}
 }