Add mapping options to CLI (e.g. --install-deps)

[Nour-Mws]

This option is already usable via the toml config or env variables as per #252 but is still not available through the CLI.
This issue is to be tackled after some more progress and visibility have been obtained on #256.

Collecting here pieces of feedback from @jherland when I attempted to add the option in #252.

I don't think the default=False is needed when action="store_true" is used. Furthermore, I think it actually defeats the configuration cascade (which really depends on all relevant CLI options having default=argparse.SUPPRESS). What we want is that this behavior:

• --install-deps on the command-line causes args.install_deps == True to be returned from argparse
• No --install-deps on the command-line causes args.install_deps to NOT EXIST, allowing the install_deps option from the config file to take effect.

Additionally, I think the help message should emphasize that the deps are installed in a temporary/separate venv:

    parser.add_argument(
        "--install-deps",
        action="store_true",
        help=(
            "Allow FawltyDeps to `pip install` declared dependencies into a"
            " separate temporary virtualenv to discover the imports they expose"
        ),
    )

Originally posted by @jherland in #252 (comment)

This change highlights the problem that I talked about above: Setting install_deps = True in the config file, and then running without --install-deps on the command line should let the config file decide, but this illustrates that it doesn't: the default=False will always override whatever comes from the config file...

However, I realize there is an inherent problem with boolean command-line flags here: If you can only enable some behavior with a command-line option, then - if the behavior has already been enabled in the config file - how could you then disable this behavior at the command-line? I think the answer here is to provide a corresponding (mutually exclusive) --no-install-deps option that sets install_deps to False.

In other words we should have:

• --install-deps: Set install_deps to True
• --no-install-deps: Set install_deps to False
• (nothing): Don't change install_deps at all. Use the value from the config file, falling back to the ultimate hardcoded default (False)

Originally posted by @jherland in #252 (comment)