Specify a custom decoder

[razor-1]

This is mostly to support claim types other than MapClaims, but may have other uses as well. Made it a public struct member instead of changing the NewWithParser signature.

A simple example custom decoder might be:

func MyDecoder(tokenString string) (t *jwt.Token, err error) {
    t, err = jwt.ParseWithClaims(tokenString, &MyCustomClaims{}, func(token *jwt.Token) (interface{}, error) {
        return []byte("mysecretkey"), nil
    })
    return
}

[razor-1]

Are PRs accepted here? any feedback on this? thank you