New apt-env

This is new apt environment script to download Debian packages in a sandbox. This is just a new improvement of Iang's script. The new features are:

• Using source parts (sources.list.d directory)
  This is common in Ubuntu. Personally, I think it's a good practice to separate main package sources and additional package sources.
• Optional apt configuration.
  In apt.conf, I've added clues how to add package cacher like apt-cacher-ng and also how to download different architecture so you don't have to look at the man page for this common things (at least for me).
• Better prompt
  Now to prompt is not messy any longer. The bug has been fixed
• Autocomplete is now possible. I added . /etc/bash_completion if the file exists

I've attached the script below (change the file back to "apt-env")

Later, I'd like to push this enhancement to Iang's github repository.

Enjoy the script!