Add the ability to do etcd snapshot to a pipe or a named pipe as a pseudo file

[Ais8Ooz8]

What would you like to be added?

An additional feature is needed within the etcdctl snapshot. For example, save command saves to a file, pipe command sends to stdout.

Why is this needed?

For security reasons, it may be preferable not to save etcd data to disk, but only to keep it in memory and pass it to a subsequent command.