Set bundles paths when doing a build

Only if using an alternative dist directory, set the bundles path during the build so that the user only needs to set the path to their main bundle entry point. So when using an alternate path it writes this to the bundle:

```js
System.paths["bundles/*.css"] ="other_dist/bundles/*css";
System.paths["bundles/*"] = "other_dist/bundles/*.js";
```

Which overwrites the default settings in steal.js. The user still has to set the path to their main entry point bundle.