Support AttachOptions via new RunCommand

```go
c.RunCommand([]string{"/bin/sh", "ls"}, &AttachOptions{
	Stdinfd: os.Stdin.Fd(),
	Stdoutfd: os.Stdout.Fd(),
	Stderrfd: os.Stderr.Fd(),
	Cwd: "/tmp",
	Env: []string{"FOO=BAR"},
})
```
Merge pull request #18 from dropbox/attach-options

Support AttachOptions via new RunCommand