Add a rackup option for a one-liner rack app server

Hi, Rack team,

I have created a rackup option `-b/--builder`, which is passed with `Rack::Builder`'s build script.

This patch allows you a command like below, and will boot up the file directory server, without any `config.ru` file.

```bash
bundle exec rackup -b "run Rack::Directory.new '.'"
```

I intend this option to be just used as `plackup`'s one-liner option `-e`.

How do you think?

Thanks,
KONDO Uchio