Also allow string values for rules

Rules can have string values, and the type annotations should allow this
to avoid type analysis warnings in code using this library.

Author: oliverklee

src/Rule/Rule.php

@@ -24,7 +24,7 @@ class Rule implements Renderable, Commentable
     private $sRule;
 
     /**
-     * @var RuleValueList|null
+     * @var RuleValueList|string|null
      */
     private $mValue;
 
@@ -171,15 +171,15 @@ public function getRule()
     }
 
     /**
-     * @return RuleValueList|null
+     * @return RuleValueList|string|null
      */
     public function getValue()
     {
         return $this->mValue;
     }
 
     /**
-     * @param RuleValueList|null $mValue
+     * @param RuleValueList|string|null $mValue
      *
      * @return void
      */