powerpill completion

[In-line]

I tried to add implement this on my own. I think this should be combination of _gnu_generic and _pacman, where they complement each other, but I lack skill in shell scripting to getting over it.

Output of powerpill --help. So you can see it's basically same with the exception of 2 more arguments.

USAGE
  Powerpill.py [Powerpill.py options] [pacman args]

OPTIONS
  Powerpill.Py should accept the same arguments as Pacman, e.g.

      Powerpill.py -Syu

  See "pacman --help" for further help.

  The following additional arguments are supported:

    --powerpill-config <path>
        The path to a Powerpill configuration file.
        Default: /etc/powerpill/powerpill.json

    --powerpill-clean
        Clean up leftover .aria2 files from an unrecoverable download. Use this
        option to resolve aria2c length mismatch errors.

Temporary solution can be found in: ohmyzsh/ohmyzsh#2104
I understand that _pacman is sufficient for most cases, but I think supporting everything is better.