Ability to decode non predefined fields

[yogsototh]

I might have missed something. But I don't see how I can get claims not already predefined in JwtClaims (typically email claim from OIDC from Google for example).
A quick fix would be to simply change the type of decodeClaims or to add another function like this:

decodeClaims :: ByteString
             -> Either JwtError (JwtHeader, JwtClaims)
decodeClaims = decodeJwt

decodeJwt :: (FromJSON a) => ByteString
          -> Either JwtError (JwtHeader, a)
decodeJwt jwt = do
    let components = BC.split '.' jwt
    when (length components /= 3) $ Left $ BadDots 2
    hdr    <- B64.decode (head components) >>= parseHeader
    claims <- B64.decode ((head . tail) components) >>= parseClaims
    return (hdr, claims)
  where
    parseClaims bs = maybe (Left BadClaims) Right $ decodeStrict' bs

So I will be able to get any claim easily.
Another solution would be to change the type of JwtClaims to contains a Map for all non predefined claims.

If you believe this is reasonable I'll make a quick PR.

[tekul]

Included in version 0.8.0.

[yogsototh]

thanks!