monguard should optionally accept a filepath for a CRL representing a certificate revocation list. We should validate that crlFile is only specified if caFile is also present as those two params control whether or not monguard verifies client certificates when they are presented. This should be propagated into OpenSSL if specified. We should then test that certificates specified in the CRL are rejected when presented by clients.

Also, test that invalid filepaths for CRLs, caFile, certificateFile, and privateKeyFile are all handled gracefully and do not cause monguard to exit in a panic.