Update Avatar.php (#75)

Added a mutator for the chars attribute. This allows more control and variability over the initials length. Think of companies names that are composed of variable number of words.

src/Avatar.php

@@ -273,6 +273,13 @@ public function setShape($shape)
 
         return $this;
     }
+
+    public function setChars($chars)
+    {
+        $this->chars = $chars;
+
+        return $this;
+    }
 
     public function getInitial()
     {