Checking that token is expired

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Hello,
Why not add a new method like JWT.expired?(token)? Otherwise we have to decode the token which raises exception. But with such a method we could check from anywhere whether or not a token is expired. I already read this discussion #124.

[excpt]

Thinking about Version 3.0.

Changing the api to use methods like .decode, .decode!, .encode and .encode!.

That gives us the space to implement a method like .expired?() or .valid?() on the .encode() or .decode() results.

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@excpt Is work in progress, or can I give you a hand?