Add Option minHeaderLevel

Add a new option to define the minimal Header level to be used
e.g. if the option `->minHeaderLevel(3)` a header defined with a single hash `# heading` will be parsed as `<h3>heading</h3>`

Parsedown.php

@@ -83,6 +83,15 @@ function setUrlsLinked($urlsLinked)
 
     protected $urlsLinked = true;
 
+    function minHeaderLevel($minHeaderLevel)
+    {
+        $this->minHeaderLevel = (int) $minHeaderLevel;
+
+        return $this;
+    }
+
+    protected $minHeaderLevel;
+
     function setSafeMode($safeMode)
     {
         $this->safeMode = (bool) $safeMode;
@@ -540,6 +549,8 @@ protected function blockHeader($Line)
     {
         $level = strspn($Line['text'], '#');
 
+        $level = (isset($this->minHeaderLevel) ? $this->minHeaderLevel + $level - 1 : $level);
+
         if ($level > 6)
         {
             return;