support a secretProvider

[kevinranks]

Hello,

I hope you accept this simple PR that adds support for a secretOrKeyProvider as documented at passport-jwt#usage.

The change simply checks if the passed secret is a function or not. It will then it will set the appropriate key (secretOrKey or secretOrKeyProvider) as defined in passport-jwt.

Thanks!

[daffl]

This makes sense, thank you. Could you also add the option in the documentation. How would a test for this look like?

[kevinranks]

@daffl Can you rebuild this? Not sure why the client tests failed...