config.get('foo.bar.baz') does not process intermediate templates in path

When getting a config "path" (passing e.g. `"foo.bar.baz"` or `['foo', 'bar', 'baz']` to `grunt.config.get`) grunt doesn't process templates as it's stepping through the object graph.

``` js
grunt.initConfig({
getFooBarBaz: '<%= foo.bar.baz %>',
foo: '<%= _foo %>',
_foo: {
bar: '<%= _bar %>'
},
_bar: {
baz: 123
}
});
grunt.config('foo');
// returns { bar: { baz: 123 } }

grunt.config('foo.bar.baz');
grunt.config('getFooBarBaz')
// Both return { bar: { baz: 123 } } but I expected 123
```

Start by reproducing the issue with the grunt.initConfig and grunt.config examples in the report, then trace how grunt.config resolves dotted and array paths. Done means intermediate templates are processed while traversing the object graph so both reported lookups return 123, with coverage for the shown cases.

Written by the indexing model from the issue text.