Make - read from stdin for rm and rmi

Modify Command.go so that if we pass `-` on the command line docker rm and docker rmi also try and remove images recieved on stdin as a list of images/containers one per line

This matches the syntax used in other linux programs, such as xz.

This is a partial fix for #928

I'm pretty sure there should be some basic tests added. But I'm not seeing anywhere that explicitly tests commands. Am I missing a test file or are we not doing tests for commands?