Imports of crlcheck command

PathSynopsis
flagPackage flag implements command-line flag parsing.
fmtPackage fmt implements formatted I/O with functions analogous to C's printf and scanf.
github.com/google/certificate-transparency-go/x509Package x509 parses X.509-encoded keys and certificates.
github.com/google/certificate-transparency-go/x509utilPackage x509util includes utility code for working with X.509 certificates from the x509 package.
osPackage os provides a platform-independent interface to operating system functionality.