Add ParseOption in order to skip claim validation in jwt.Parse method

[re-Tick]

I am trying to test my API's which are having different expiration time, so I had written my custom MiddlewareFunc. Claims will be validated in this custom function that's need to avoid validation in ParseWithClaims method.
If you approve this, I will make a PR for this.

[re-Tick]

I have made a PR for this issue. These changes will not effect existing flow. They just adds ParseOptions.
Please review it.