public function configure($key, $value=null)
    {
        if ($this->wasBooted()) {
            throw new UnsupportedUsageException('You can only set configuration before boot.');
        }

        if (!is_array($key)) {
            $this->config[$key] = $value;
            return $this;
        }

        foreach ($key as $configKey=>$value) {
            $this->configure($configKey, $value);
        }

        return $this;

    }