Implement basic tab completion support

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

commands/compose/abort.go

@@ -6,11 +6,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 )
 
+type Abort struct{}
+
 func init() {
-	register("abort", CommandAbort)
+	register(Abort{})
 }
 
-func CommandAbort(aerc *widgets.Aerc, args []string) error {
+func (_ Abort) Aliases() []string {
+	return []string{"abort"}
+}
+
+func (_ Abort) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Abort) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 1 {
 		return errors.New("Usage: abort")
 	}

commands/compose/compose.go

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

commands/compose/edit.go

@@ -6,11 +6,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 )
 
+type Edit struct{}
+
 func init() {
-	register("edit", CommandEdit)
+	register(Edit{})
 }
 
-func CommandEdit(aerc *widgets.Aerc, args []string) error {
+func (_ Edit) Aliases() []string {
+	return []string{"edit"}
+}
+
+func (_ Edit) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Edit) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 1 {
 		return errors.New("Usage: edit")
 	}

commands/compose/next-field.go

@@ -7,16 +7,21 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/widgets"
 )
 
+type NextPrevField struct{}
+
 func init() {
-	register("next-field", NextPrevField)
-	register("prev-field", NextPrevField)
+	register(NextPrevField{})
 }
 
-func nextPrevFieldUsage(cmd string) error {
-	return errors.New(fmt.Sprintf("Usage: %s", cmd))
+func (_ NextPrevField) Aliases() []string {
+	return []string{"next-field", "prev-field"}
 }
 
-func NextPrevField(aerc *widgets.Aerc, args []string) error {
+func (_ NextPrevField) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ NextPrevField) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) > 2 {
 		return nextPrevFieldUsage(args[0])
 	}
@@ -28,3 +33,7 @@ func NextPrevField(aerc *widgets.Aerc, args []string) error {
 	}
 	return nil
 }
+
+func nextPrevFieldUsage(cmd string) error {
+	return errors.New(fmt.Sprintf("Usage: %s", cmd))
+}

commands/compose/send.go

@@ -20,13 +20,23 @@ import (
 	"git.sr.ht/~sircmpwn/aerc/worker/types"
 )
 
+type Send struct{}
+
 func init() {
-	register("send", SendMessage)
+	register(Send{})
 }
 
-func SendMessage(aerc *widgets.Aerc, args []string) error {
+func (_ Send) Aliases() []string {
+	return []string{"send"}
+}
+
+func (_ Send) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Send) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) > 1 {
-		return errors.New("Usage: send-message")
+		return errors.New("Usage: send")
 	}
 	composer, _ := aerc.SelectedTab().(*widgets.Composer)
 	config := composer.Config()