Support importing existing flag package flags

[jbeda]

This is important for packages (like https://github.com/golang/glog) where the flags are embedded deeply into the code.

I already have this supported in kubernetes here that we can borrow/steal: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/util/plog_import.go

(I typo'd the name of that file and I'll fix it up soon).

This would be a lot easier if the pflag.Value.Type didn't exist or was an extended optional interface. It is unclear why that is there and where it is used.

[ogier]

pflag.Value.Type isn't something I'm aware of. I think this is something @spf13 added, but adding methods to Go interfaces is a pretty strong backwards incompatibility because it can make existing types no longer satisfy interfaces they expect to.

I agree that this should be an additional optional interface if it exists at all.