Add pre and post task lists into help output

[tyzhnenko]

Example output

❯ inv --help main-task
Usage: inv[oke] [--core-opts] main-task [other tasks here ...]

Docstring:
  none

Options:
  none

Pre-tasks:
  pre-task
  pre-task-2

Post-tasks:
  none

Issue: #860

[antoinebrl]

Instead of displaying none, can the sections "Pre-tasks" and "Post-tasks" be made optional if the task doesn't define pre- and post- tasks respectively?

[tyzhnenko]

@antoinebrl sure, on the other hand, other sections have similar behavior with 'none' in the output. Won't it be confusing to have this difference?