`preHook`/`postHook`: change type from str to lines for composability?

[kfollesdal]

preHook and postHook are currently typed as str. Would types.lines be a better fit for the composable wrapper model?

That would allow modules to append shell fragments to preHook/postHook, even when those hooks are already set in a base module.

If interesting, I can make a PR for this.