add group without prefix;change verify method;

1.migrations
```
  $table->string('key', 50)->unique();
```
2.Option.php
```
private function verify($key = '')
    {
        if (empty($key) || !is_string($key)) {
            throw new InvalidArgumentException('Invalid Option Key!');
        }
        $key = e(trim($key));
        $key = strtolower($key);
        $key = $this->toPrefix($key);
        if (strlen($key) > 50) {
            throw new InvalidArgumentException('Invalid Option Key Length, max=50!');
        }
        return $key;
    }

    private function toPrefix($key)
    {
        $prefix = \Config::get('option::default_prefix');
        if (empty($prefix)) {
            throw new Exception('default_prefix can not be empty');
        }
        if (!str_is('*.*', $key)) {
            $key = sprintf('%s.%s', $prefix, $key);
        }
        return $key;
    }
```
3.groupWithoutPrefix
```
 public function getGroup($group, $withPrefix = true)
    {
        $all    = $this->all();
        $prefix = $group . '.';
        if ($withPrefix) {
            foreach ($all as $key => $value) {
                if (!starts_with($key, $prefix)) {
                    unset($all[$key]);
                }
            }
        } else {
            $newAll = [];
            foreach ($all as $key => $value) {
                if (starts_with($key, $prefix)) {
                    $newKey          = preg_replace("#^$prefix#", '', $key, 1);
                    $newAll[$newKey] = $value;
                }
            }
            $all = $newAll;
        }
        return count($all) > 0 ? $all : null;
    }
```