benchmark: allow multiple values for same config

This allows running a benchmark with two or more values for the same
config rather than just one or all of them, for example:

```
node benchmark/buffers/buffer-creation.js type=buffer() type=fast-alloc type=fast-alloc-fill
```

PR-URL: https://github.com/nodejs/node/pull/11819


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>