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
{db, repoconfmanager}.py: Improves error return handling

Previously repoconfmanager would not take into account that it
could get multiple return booleans from the different ConfigHandler
classes that it calls, this commit adds that awareness to the class
along with reflections of the change in db.py to interact with the
multiple returns that it might get from the RepoConfManager class.
cli.py: Fixes incorrect determination of supported overlay

For cli output of overlay info when running --info or --list, you
would previously get incorrect output telling you the overlay you
are listing isn't supported. This was due to the fact that the
boolean used to determine whether it was supported or not during
the unpacking of the overlay info was actually the boolean that
determined if the overlay was official or not.
repoconfmanager.py: modifies str checking

X-Gentoo-Bug: 518846
X-Gentoo-Bug-URL: https://bugs.gentoo.org/518846
cli.py: Removes unncessary comma placement

To add coherence to the layman output, messages have been altered
and unnecessay commas have been removed from the CLI messages.
Fix format string in _verify_overlay_type

Signed-off-by: Devan Franchini <twitch153@gentoo.org>

X-Gentoo-Bug: 519852
X-Gentoo-Bug-URL: https://bugs.gentoo.org/519852
Fixes hard-coded defaults and EPREFIXIES repos_conf

X-Gentoo-Bug: 519922
X-Gentoo-Bug-URL: https://bugs.gentoo.org/519922
Merge pull request #18 from twitch153/bug-squashing

This merge attempts to squash the following Gentoo bugs:
   - X-Gentoo-Bug: 315359
   - X-Gentoo-Bug: 396031
   - X-Gentoo-Bug: 461540
   - X-Gentoo-Bug: 518846
   - X-Gentoo-Bug: 519922
Centralizes common archive overlay code

This commit brings archive overlay code to one common place, in
an attempt to make maintaining and adding archive overlay types
to layman much easier.

config.py: Alters the clean_tar option to clean_archive.

tar.py: Removes doctest tests from tar.py as there is already an
existing external.py test.
Migrates run_command to utily.py
Adds layman-mounter utility script

api.py: Adds Mounter() initialization in the LaymanAPI.
constants.py: Adds MOUNT_TYPES to constants to keep track of
mountable overlay classes.
archive.py: Adds check to unmount overlays if they are being
synced.
Adds Squashfs overlay support

constants.py: Adds Squashfs overlay type to MOUNT_TYPES constants.
Adds Squashfs overlay external tests