feat: Add `--output` to allow user to show contents of JWT tokens

[RiddleMan]

Would be great to have a ability to look into the contents of JWT token if --output json is passed. If token isn't a JWT, then show an error.

[RiddleMan]

jsonwebtoken library requires to verify token with the right key before we could get all claims. It's right approach, but might need JWKS and it depends on #35