Add Handler priority constants

src/DamnIt/Handler/Handler.php

@@ -10,6 +10,17 @@ use DamnIt\Run;
 
 class Handler implements HandlerInterface
 {
+    /**
+     * Can be used as priorities with Run::addHandler,
+     * to ensure the handler is added to the end or
+     * beginning of the handler stack.
+     *
+     * @example Low priority handler
+     *     $run->addHandler($myHandler, Handler::LOW_PRIORITY);
+     */
+    const LOW_PRIORITY  = 0x10;
+    const HIGH_PRIORITY = 0x20;
+
     /**
      * @var DamnIt\Run
      */