Expose `buildCmd`

I don't know how much spread is this need. In my specific case, I need to access the underlying `exec.Cmd` that is built in `buildCmd` without starting it. Currently, all the exposed functions that return `icmd.Result` already started the underlying `exec.Cmd`.

In case you wonder why I need to do this, here's some context. I need to test an interactive REPL, so I'm using [this](https://github.com/kr/pty). The entrypoint of `pty` is this function:
```
func Start(c *exec.Cmd) (pty *os.File, err error)
```
so I'd like to do something like:
```
r := icmd.BuildCmd(cmd)
in, err := pty.Start(r.Cmd)
```