allow pods in pools to be bootstrapped when created

When using pools it's mostly for performance reason.
Often pods require additional bootstrapping before you can run what you want to run (e.g. requiring `clojure.test` or `garden.core`).
This requiring can take a significant amount of time and for that reason it'd be nice to allow this to be part of the pool lifecycle.

I was able to reduce `garden` compilation times down to ~0.5s from ~3s:

```clojure
(defn garden-pool []
  (pod/pod-pool 2 
                (update-in (boot/get-env) [:dependencies] conj '[garden "1.2.5"])
                (fn [pod] (pod/require-in pod 'garden.core))))
```

Probably the code can be improved, let me know what could be done better.
I will also add some docstrings to the pod-pool function before merging this (if we do).

**EDIT** I didn't really try to break it so potentially there are bugs. Beware :)