Implement abort command

This allows the user to close the compose tab without sending their
current composition.

commands/compose/abort.go

@@ -0,0 +1,23 @@
+package compose
+
+import (
+	"errors"
+
+	"git.sr.ht/~sircmpwn/aerc2/widgets"
+)
+
+func init() {
+	register("abort", CommandAbort)
+}
+
+func CommandAbort(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 1 {
+		return errors.New("Usage: abort")
+	}
+	composer, _ := aerc.SelectedTab().(*widgets.Composer)
+
+	aerc.RemoveTab(composer)
+	composer.Close()
+
+	return nil
+}