Pre- and post-activation hooks.

This change adds user-scoped activate and deactivate hook scripts. If the following scripts exist, they will be sourced automatically at the appropriate times:

* ~/.hsenv/bin/pre-activate
* ~/.hsenv/bin/post-activate
* ~/.hsenv/bin/pre-deactivate
* ~/.hsenv/bin/post-deactivate

I use this to customize my prompt, for example.