WIP: Allow configuration of attach_options (env, cwd)

This isn't complete, but I'm interested to hear feedback. Creating the Command struct was the cleanest way I could see to introduce this without adding a lot of extra complexity (and without butchering the existing APIs).

See the included tests for usage, but the gist is:

```go
cmd := &Command{
    Args: []string{"/bin/sh", "-c", "exit 0"},
    Cwd: "/tmp",
    Env: []string{"FOO=BAR"},
    ClearEnv: true,
}
cmd.Run(container)
```

This extends the existing patch in GH-10