load(): pass instance to callback function

if you do not retain the value returned by `rc()`, it's effectively impossible to get at the `path` if wrapping this function in the Q library:

```js
// before this change:
Q.nfcall(require('cli-rc'))
  .then(function(config) {
    // now you have your data, but since the wrapper returns a Promise instead of a RunControl instance,
    // we must use a callback if we want it.
  });
```

```js
// after:
Q.nfcall(require('cli-rc'))
  .spread(function(config, rc) {
    // now we have access to the instance, and thusly the path property.
  });
```
Merge pull request #6 from boneskull/master

load(): pass instance to callback function