add utility for binding flags and building api server clients

This utility provides one stop shopping for a command line executable to get bind the correct flags, build the client config, and create a working client.  The code for usage reduces to something like this:

	authLoader := &PromptingAuthLoader{os.Stdin}
	clientBuilder := clientcmd.NewClientBuilder(authLoader)
	clientBuilder.BindFlags(cmds.PersistentFlags())
	apiClient, err := clientBuilder.BuildClient()

I made the flags congruent with kubectl and used kubectl as an example spot for usage.  This makes use of the auth config work in #2340 and the Bind and Build approach could be used to unify client creation and flag names.

@smarterclayton You've had an interest in this area.
I'm skeptical that clients other than kubectl (non-interactive ones) will want to use persistent flags.  And, in the case of kubectl, the two lines in question are right next to each other.