[WIP] Refactor classes postrun_command, prerun_command

[dhoppe]

This pull request will prepare the module for data-in-modules and provide a REFERENCE.md. Unfortunately this module uses a lot of classes and I need to figure out which one are private. Because some variables are named or used badly, this change will be backwards incompatible and end up in a major release.

[vox-pupuli-tasks[bot]]

Dear @dhoppe, thanks for the PR!

This is Vox Pupuli Tasks, your friendly Vox Pupuli GitHub Bot. I noticed that your pull request contains merge conflict. Can you please rebase?

You can find my sourcecode at voxpupuli/vox-pupuli-tasks