Added Certificate.CertPEM() and Certificate.KeyPEM() methods as convenience methods to return the certificate and key in PEM format for using in single-value context, for example when populating struct field. They are alternative to the Certificate.PEM() method, which returns the certificate, key, and error.

c := Certificate{Subject: "CN=Joe"}

var foo = struct {
	cert []byte
	key  []byte
}{
	cert: c.CertPEM(),
	key:  c.KeyPEM(),
}

If there are errors, the convenience methods cannot return error code. They will call panic() instead.