Feature: Suppress ignored vulnerabilities

[andy-maier]

Description

If vulnerabilities have to be ignored for some reason (e.g. because the fixed version of the dependent package can only be used on a subset of Python versions), the report of the current version of safety still reports them. That lets any newly found vulnerabilities "drown" amongst all the ignored vulnerabilities.

I suggest that a policy file parameter gets introduced that controls whether or not ignored vulnerabilities are included in the report.