Add a configuration option for ignoring expirations.

[chrisgavin]

This adds a new configuration option ignore_expirations, which if set, will cause Entitlements to ignore expiration dates and treat everything as not being expired.

The main reason you might want to do this is during CI. Say for example you have a CI check that asserts certain files are not empty, i.e. they have at least one user included. You might want this CI check to ignore expirations, because you don't want the check to start failing on an arbitrary date when the expiration passes.