Feature request: pre-init hook

[niwinz]

Would be awesome to have the pre-init hook.

Why? (Rationale): Right now we have prep-key and pre-init-spec, on prep-key you still have all refs unresolved. On my use case, some refs will resolve to nil because they are "dynamic" and depends on environment variables. I want to have the option to remove all keys with nil values before passing it to pre-init-spec.

Un summary: a hook that is executed after prep-key but before pre-init-spec, with refs already resolved.

It makes sense for you?