Add Duration type support

[bigBron]

conf.Duration(key) = time.Duration
---
Save Steps：
  timeout := time.Duration(Conf.Int64("timeout"))
  timeoutDuration := time.Second * timeout 

[inhere]

hi @bigBron, welcome to send a PR for this feature. :)

[inhere]

hi @bigBron The feature released on https://github.com/gookit/config/releases/tag/v2.1.8