WIP: add unload hook

[skogsbrus]

This is a WIP proposal for adding an unload behavior to direnv, as discussed in #129. Mostly intended as a conversation starter on how this should be implemented instead.

This implementation is very hacky:

• Allows circumvention of 'direnv allow', since the unload hook can source another script which can be modified without 'direnv allow' being required to run
• Does not introduce any new abstraction for user-provided hooks; instead relies on the hook command being exported as an env variable and then evaluated
• Defining long unload hooks is cumbersome without sourcing another script
• Seems to also be executed on 'direnv deny', which I guess makes sense

Sample output:

johanan@airm2 (add-unload-hook) ~/code/direnv cd foo                                      ~/code/direnv
direnv: loading ~/code/direnv/foo/.envrc
johanan@airm2 (add-unload-hook) ~/code/direnv/foo cat .envrc                          ~/code/direnv/foo
export DIRENV_UNLOAD="echo 'Execute unload hook'"
johanan@airm2 (add-unload-hook) ~/code/direnv/foo cd ..                               ~/code/direnv/foo
direnv: unloading
Execute unload hook

.envrc samples:

export DIRENV_UNLOAD="source .unloadrc"

export DIRENV_UNLOAD="echo 'Executing unload hook'"