Add method to regenerate story/mission elements based on events

[renatomassaro]

Possible interface:

• handle_file_loss cracker.file_id

which translates to:

• Listen to any "file loss" event (FileDeleted, FileEncrypted, FileHidden)
• Reacts by calling regenerate/2

The full signature, which allows for custom logic, is something like:

story_listen cracker.file_id, FileDeletedEvent, :on_file_delete

callback on_file_delete [, event] do
  {:regenerate, []}
end

The end result is the same: the regenerate/2 is called. regenerate must be able to:

• figure out what is inconsistent;
• fix it (delete everything and recreate; or only recreate what is inconsistent. Depends on the implementation of each step/mission)

Except for the helper handle_file_loss, the listener interface described above is already implemented. Now it's just a matter of figuring out how to create the regenerate/2 method in a maintainable way.

[renatomassaro]

:regenerate and :fail actions have been merged into :restart.

PR #383 solves this in a scalable way, but I've postponed the creation of more specific listeners like handle_file_loss, handle_file_unavailability, handle_log_loss, handle_log_misedit etc.