README, _cli: Add a -S, --strict mode

[woodruffw]

Closes #142.

[woodruffw]

This doesn't play completely nicely with the context manager we use for the spinner:

root@1aa0ddda304c:/tmp/pip-audit# pip-audit -S
WARNING:pip_audit._service.pypi:Warning: pip 20.0.2 doesn't support the `cache dir` subcommand, unable to reuse the `pip` HTTP cache and using "/root/.pip-audit-cache" instead
/ Auditing pyparsing (3.0.6)            Fatal: python-apt: Dependency not found on PyPI and could not be audited: python-apt (2.0.0+ubuntu0.20.4.6)

WARNING:pip_audit._service.pypi:Warning: Dependency not found on PyPI and could not be audited: python-apt (2.0.0+ubuntu0.20.4.6)

(Note that the spinner and the Fatal: share a line.)

Working on that now.

[woodruffw]

Reusing our log buffering work does the trick:

root@1aa0ddda304c:/tmp/pip-audit# pip-audit -S
WARNING:pip_audit._service.pypi:Warning: pip 20.0.2 doesn't support the `cache dir` subcommand, unable to reuse the `pip` HTTP cache and using "/root/.pip-audit-cache" instead
/ Auditing pyparsing (3.0.6)
WARNING:pip_audit._service.pypi:Warning: Dependency not found on PyPI and could not be audited: python-apt (2.0.0+ubuntu0.20.4.6)
ERROR:pip_audit._cli:Fatal: python-apt: Dependency not found on PyPI and could not be audited: python-apt (2.0.0+ubuntu0.20.4.6)
root@1aa0ddda304c:/tmp/pip-audit# exit

The warning and error are dupes, but that's mostly cosmetic. We could convert the warning into a debug-level message, so that it doesn't show up at the default loglevel.

Edit: Done.

[tetsuo-cpp]

LGTM!