added threshold command implementation

Author: fariasmaiquita

Command.php

@@ -638,6 +638,17 @@ public function transverse(): self
         return $this;
     }
 
+    /**
+     * @see http://www.imagemagick.org/script/command-line-options.php#threshold
+     */
+    public function threshold(string $threshold): self
+    {
+        $this->command[] = '-threshold';
+        $this->command[] = $this->ref->threshold($threshold);
+
+        return $this;
+    }
+
     /**
      * /!\ Append a raw command to ImageMagick.
      * Not safe! Use at your own risks!