use an interface for Cmd

[mischief]

it would be great if this package could utilize an interface for the subprocess. that way, a subprocess interface could be implemented on top of other methods of executing a subprocess, such as a pure go ssh client, executing commands on a remote machine.

we use an interface for commands in our project at https://github.com/coreos/mantle/blob/master/system/exec/exec.go#L32 so something similar in gexpect would be fantastic.