Implement basic tab completion support

Tab completion currently only works on commands. Contextual completion
will be added in the future.

commands/msg/archive.go

@@ -18,11 +18,21 @@ const (
 	ARCHIVE_MONTH = "month"
 )
 
+type Archive struct{}
+
 func init() {
-	register("archive", Archive)
+	register(Archive{})
 }
 
-func Archive(aerc *widgets.Aerc, args []string) error {
+func (_ Archive) Aliases() []string {
+	return []string{"archive"}
+}
+
+func (_ Archive) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Archive) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 2 {
 		return errors.New("Usage: archive <flat|year|month>")
 	}

commands/msg/copy.go

@@ -11,12 +11,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
 )
 
+type Copy struct{}
+
 func init() {
-	register("cp", Copy)
-	register("copy", Copy)
+	register(Copy{})
 }
 
-func Copy(aerc *widgets.Aerc, args []string) error {
+func (_ Copy) Aliases() []string {
+	return []string{"copy"}
+}
+
+func (_ Copy) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Copy) Execute(aerc *widgets.Aerc, args []string) error {
 	opts, optind, err := getopt.Getopts(args, "p")
 	if err != nil {
 		return err

commands/msg/delete.go

@@ -10,12 +10,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
 )
 
+type Delete struct{}
+
 func init() {
-	register("delete", DeleteMessage)
-	register("delete-message", DeleteMessage)
+	register(Delete{})
 }
 
-func DeleteMessage(aerc *widgets.Aerc, args []string) error {
+func (_ Delete) Aliases() []string {
+	return []string{"delete", "delete-message"}
+}
+
+func (_ Delete) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Delete) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 1 {
 		return errors.New("Usage: :delete")
 	}

commands/msg/move.go

@@ -11,12 +11,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
 )
 
+type Move struct{}
+
 func init() {
-	register("mv", Move)
-	register("move", Move)
+	register(Move{})
 }
 
-func Move(aerc *widgets.Aerc, args []string) error {
+func (_ Move) Aliases() []string {
+	return []string{"mv", "move"}
+}
+
+func (_ Move) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Move) Execute(aerc *widgets.Aerc, args []string) error {
 	opts, optind, err := getopt.Getopts(args, "p")
 	if err != nil {
 		return err

commands/msg/msg.go

@@ -8,9 +8,9 @@ var (
 	MessageCommands *commands.Commands
 )
 
-func register(name string, cmd commands.AercCommand) {
+func register(cmd commands.Command) {
 	if MessageCommands == nil {
 		MessageCommands = commands.NewCommands()
 	}
-	MessageCommands.Register(name, cmd)
+	MessageCommands.Register(cmd)
 }

commands/msg/read.go

@@ -10,12 +10,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
 )
 
+type Read struct{}
+
 func init() {
-	register("read", Read)
-	register("unread", Read)
+	register(Read{})
 }
 
-func Read(aerc *widgets.Aerc, args []string) error {
+func (_ Read) Aliases() []string {
+	return []string{"read", "unread"}
+}
+
+func (_ Read) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Read) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 1 {
 		return errors.New("Usage: " + args[0])
 	}

commands/msg/reply.go

@@ -18,12 +18,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 )
 
+type reply struct{}
+
 func init() {
-	register("reply", Reply)
-	register("forward", Reply)
+	register(reply{})
 }
 
-func Reply(aerc *widgets.Aerc, args []string) error {
+func (_ reply) Aliases() []string {
+	return []string{"reply", "forward"}
+}
+
+func (_ reply) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ reply) Execute(aerc *widgets.Aerc, args []string) error {
 	opts, optind, err := getopt.Getopts(args, "aq")
 	if err != nil {
 		return err