Allow passing through safety args, to work around pyup unreliability

[lahwran]

The issue

#2825

This issue has not been fully solved by the ability to provide a key - pyup sometimes goes down, and it usually takes a long time. Since pipenv check is the only would-be-fast way to test if a pipenv install is needed, this limits its usability.

The fix

This pull request adds a couple more arguments to pass through to safety, exposing its existing configuration options. It also adds an option to completely skip invoking safety, for cases were package installation checks and package security checks need to be run separately.

Because it's a trivial change, I also changed the return code to 2 on safety failure.

[lahwran]

I was distracted when I wrote this and I both didn't write it very well and didn't notice. My bad! I'll leave it here as inspiration.