add 2 Init lifecycle hooks

[behrica]

Add 2 post and pre initialize hooks, see here:

https://clojurians.zulipchat.com/#narrow/stream/215609-libpython-clj-dev/topic/auto-setup.20of.20venvs.20.3F/near/313150680

I tried these to setup / validate a venv and find this rather convenient. If the 2 new keys are not present in python.edn nothing happens

[cnuernber]

Sorry - I like this pathway and thought I had merged it earlier