Add customizable greeting to index page

Allow an organization to copy the greeting.html file out of the themes
directory, customize it, then point the greeting.file config option at
it so that it will survive Inbucket upgrades.

config/config.go

@@ -25,7 +25,7 @@ type SmtpConfig struct {
 type Pop3Config struct {
 	Ip4address     net.IP
 	Ip4port        int
-	Domain string
+	Domain         string
 	MaxIdleSeconds int
 }
 
@@ -35,6 +35,7 @@ type WebConfig struct {
 	TemplateDir   string
 	TemplateCache bool
 	PublicDir     string
+	GreetingFile  string
 }
 
 type DataStoreConfig struct {
@@ -328,6 +329,13 @@ func parseWebConfig() error {
 	}
 	webConfig.PublicDir = str
 
+	option = "greeting.file"
+	str, err = Config.String(section, option)
+	if err != nil {
+		return fmt.Errorf("Failed to parse [%v]%v: '%v'", section, option, err)
+	}
+	webConfig.GreetingFile = str
+
 	return nil
 }