Add support to be able to register builders

[dmikusa]

Presently, you can register buildpacks with the buildpack registry. Buildpacks are not always the first place users should go though, sometimes it's easier for users to get started by using a builder that already has sets of buildpacks packaged up together.

I think that we should expand the registry so that builder authors can register their builders. This would also allow us to build some graphs between the builders and buildpacks to show how they are being used (i.e. this buildpack is used by 5 builders, click to see a list).