Add a --privileged CLI flag

[willdurand]

This issue is related to #4306. We want to add a new CLI flag named --privileged (off/disabled by default), which will configure the linter to handle privileged extensions. If we can add it ahead of time, we can start adding it to xpi-template and xpi-manifest, even if there is no rules for privileged extensions yet. That will avoid potential issues introduced by #4306.