Add new boolean option 'extends-only' to disable built-in commands

We started to use collective.hostout, because of its extensibility (re-usable fabric-files) and buildout-configuration (using buildout's extends/<= -feature a lot), but eventually ended with a completely different/overriding command set of hostout.pushdeploy and not using any of its built-in commands:

https://github.com/datakurre/hostout.pushdeploy

Yet, because hostout already separates it's built-in commands from everything else pretty well, we would not like to completely fork it.

Therefore, this feature request. I'm also open for other ways to implement this behavior.