Provide direct access to ssh.Session.StdoutPipe

[jmptbl]

In situations where one needs to receive large amounts of binary data from a remote server, the only way to do so in vssh is for the remote server to send it in newline-terminated chunks <64k at a time.

In client.go, ssh.Session.StdoutPipe gets connected to a bufio.Scanner, which is not suitable for binary data since its default buffer is limited to 64k. Ideally there should be a way to directly export ssh.Session.StdoutPipe so that scanning and reading can be left up to the implementation that's using vssh.