docker cp should accept a file destination

Currently `docker cp` only accept path destination:

```
$ docker help cp

Usage: docker cp CONTAINER:PATH HOSTPATH

Copy files/folders from the PATH to the HOSTPATH
```

It would be useful if it was also accepting a single file destination when the source path point to a file, ex:

```
docker cp build-boot2docker:/boot2docker.iso boot2docker-$(git rev-parse HEAD).iso
```
