InstalledPackage authors should be able to provide overlays

In the current alpha, package consumers aren't able to customize package installation beyond what is exposed via data values. There should be a way to allow consumers to provide overlays via an InstalledPackage which will be applied at some point.

This may be similar to #122, but has a slight difference since package authors can't know where to inject consumer provided overlays. One idea is to just append a ytt step at the end of all other template steps which just includes the overlays, but this could suffer from some drawbacks like the overlays won't have access to data values, etc.