Add flag for google oauth2 api version

[msoderberg]

From CHANGELOG-1.10.md:

kube-apiserver: the OpenID Connect authenticator no longer accepts tokens from the Google v3 token APIs; users must switch to the "https://www.googleapis.com/oauth2/v4/token" endpoint.

If you use the token retrieved from version 3 of the api to access the k8s apiserver, kubectl will log:
error: You must be logged in to the server (Unauthorized)"
And the following will be logged in the apiserver:
E0418 12:26:47.196237 1 authentication.go:63] Unable to authenticate the request due to an error: [invalid bearer token, [invalid bearer token, invalid bearer token]]

This PR adds an "api-version" flag that defaults to "v3" for backwards compatibility. Feel free to modify it as you like before merging.

[msoderberg]

This can be closed if you merge #14 which solves the problem in a better way.

[cliveza]

👍 merge, merge it now, you can do it, you know you want to, make us all happy