Adds interactive session for unofficial overlay installs

In order to improve the communication of the risks taken from
installing unofficial overlays, a config option has been created
that when enabled will create an interactive session to confirm
that the user is aware that they are installing an unofficial
overlay.

X-Gentoo-Bug: 315359
X-Gentoo-Bug-URL: https://bugs.gentoo.org/315359
argsparser.py: Renames --setup_help to --setup-help

X-Gentoo-Bug: 461540
X-Gentoo-Bug-URL: https://bugs.gentoo.org/461540
argsparser.py: Adds support for multiple cli args

The behavior of actions such as --add have been changed to support
multiple args such as --add <ovl 1> <ovl 2>.

X-Gentoo-Bug: 396031
X-Gentoo-Bug-URL: https://bugs.gentoo.org/396031