Introduce an "envelope" package

This patch introduces an "envelope" package which, for now, provides simple
utilities for getting the user and domain of an address.

It also changes the couriers to use it (but other implementations remain, will
be moved over in subsequent patches).

internal/courier/procmail.go

@@ -8,6 +8,7 @@ import (
 	"time"
 	"unicode"
 
+	"blitiri.com.ar/go/chasquid/internal/envelope"
 	"blitiri.com.ar/go/chasquid/internal/trace"
 )
 
@@ -36,13 +37,17 @@ func (p *Procmail) Deliver(from string, to string, data []byte) error {
 	defer tr.Finish()
 
 	// Get the user, and sanitize to be extra paranoid.
-	user := sanitizeForProcmail(userOf(to))
-	tr.LazyPrintf("%s  ->  %s (%s)", from, user, to)
+	user := sanitizeForProcmail(envelope.UserOf(to))
+	domain := sanitizeForProcmail(envelope.DomainOf(to))
+	tr.LazyPrintf("%s  ->  %s (%s @ %s)", from, user, to, domain)
 
 	// Prepare the command, replacing the necessary arguments.
+	replacer := strings.NewReplacer(
+		"%user%", user,
+		"%domain%", domain)
 	args := []string{}
 	for _, a := range MailDeliveryAgentArgs {
-		args = append(args, strings.Replace(a, "%user%", user, -1))
+		args = append(args, replacer.Replace(a))
 	}
 	cmd := exec.Command(MailDeliveryAgentBin, args...)