Introduce hooks for execs

[HeavyWombat]

It would be great if there would be a way to define a custom action to be automatically triggered and done by havener once a certain action is started, for example a node exec is being executed. The hook could be something like sending a Slack message, or executing a script, or printing to a special log file. The goal should be a somewhat generic solution to be flexible.

Idea: Introduce a havener configuration file that has a section for hooks, that would execute on node-exec or pod-exec.