Handle sub-folders in bower exportOverride

Modified the `grunt bower` task such that sub-folders can be handled when specified in the `exportOverride` using curly brackets, like the `"js/{}/lang"` in this `bower.json` snippet:

```
...
    "momentjs": {
      "js": "moment.js",
      "js/{}/lang": "lang/*.js"
    },
...
```

Which would fill `main/static/src/vendor/js/momentjs/lang/` with the locale specific .js scripts.

The PR currently doesn't provide an example of the usage, but `gae-init-babel` could use the above example (with some other minor changes); just to make the intermediate structure easier to read. It does however offer derived projects a way to maintain/select certain sub-folder structures if needed.