add option to ask for a subset of the config

Allow asking for a subset of the configuration with the remaining arguments on the commandline.

Take the following cjson file, test.cjson:
```js
{
  "foo": {
    "bar": {
      "baz": "qux"
    }
  }
}
```

Running `oconf test.cjson` would yield you the entire file.

Running `oconf test.cjson foo.bar.baz` would yield `"qux"`.

Running `oconf test.cjson foo bar baz` would yield `"qux"`.

The use case is to extract configuration to command line arguments for buildProduction in assetgraph.