Mixed use support

You can now optionally configure a "no store" domain.  When inbucket
receives a message destined for a user at that domain, it will accept
the message but not store it to disk.  This allows the same instance
of Inbucket to be shared by people who wish to view email content and
those who want to load test.

config/config.go

@@ -15,6 +15,7 @@ type SmtpConfig struct {
 	Ip4address      net.IP
 	Ip4port         int
 	Domain          string
+	DomainNoStore   string
 	MaxRecipients   int
 	MaxIdleSeconds  int
 	MaxMessageBytes int
@@ -177,6 +178,15 @@ func parseSmtpConfig() error {
 	}
 	smtpConfig.Domain = str
 
+	option = "domain.nostore"
+	if Config.HasOption(section, option) {
+		str, err = Config.String(section, option)
+		if err != nil {
+			return fmt.Errorf("Failed to parse [%v]%v: '%v'", section, option, err)
+		}
+		smtpConfig.DomainNoStore = str
+	}
+
 	option = "max.recipients"
 	smtpConfig.MaxRecipients, err = Config.Int(section, option)
 	if err != nil {