code: func (c *MockConfig) Lookup(key string) (interface{}, bool) {
  	if key == "invalid" {
  		return nil, false
  	}
  	value, ok := c.Lookup(key)
  	if !ok {
  		return nil, false
  	}
  	return value, true
  }
docs: // Lookup gets the Value from config file by Key