Allow user to specify which dependencies to check

Hi, I have a case where a private package (lets call it **foo**) defines some grunt tasks as `peerDependencies`, and then another project, **bar**, defines **foo** in its `devDependencies`. Now, if in **bar** I want to use `load-grunt-tasks()`, it doesn't see the grunt tasks from **foo** even if I specify to check **foo**'s `package.json`. 

The change in this pull request makes my desired scenario possible, with code like the following inside **bar**'s `Gruntfile.js`:

```js
require('load-grunt-tasks')(grunt, 'grunt-*', require('foo/package'), ['devDependencies', 'peerDependencies']);
```

PS thanks for the useful module!