Accept optional npm config key-value pairs. This enables --save, --save-dev, etc.

This addresses issue https://github.com/boo1ean/irm/issues/1.

It will you to call `irm` in code like so:

```js
irm('./app', { save: true });
irm('./test', { 'save-dev': true });
```

or though the cli:

```bash
$ cd project/app && irm --save
$ cd project/test && irm --save-dev
```