Support default commands.

Fixes #41.

context.go

@@ -397,6 +397,24 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 			return fmt.Errorf("unexpected token %s", token)
 		}
 	}
+
+	// End of the line, check for a default command.
+	var defaultNode *Path
+	for _, child := range node.Children {
+		if child.Type == CommandNode && child.Tag.Default != "" {
+			if defaultNode != nil {
+				return fmt.Errorf("can't have more than one default command under %s", node.Summary())
+			}
+			defaultNode = &Path{
+				Parent:  child,
+				Command: child,
+				Flags:   child.Flags,
+			}
+		}
+	}
+	if defaultNode != nil {
+		c.Path = append(c.Path, defaultNode)
+	}
 	return nil
 }
 
@@ -617,6 +635,7 @@ func checkMissingChildren(node *Node) error {
 		missing = append(missing, strconv.Quote(strings.Join(missingArgs, " ")))
 	}
 
+	haveDefault := 0
 	for _, child := range node.Children {
 		if child.Hidden {
 			continue
@@ -627,10 +646,19 @@ func checkMissingChildren(node *Node) error {
 			}
 			missing = append(missing, strconv.Quote(child.Summary()))
 		} else {
+			if child.Tag.Default != "" {
+				if len(child.Children) > 0 {
+					return fmt.Errorf("default command %s must not have subcommands or arguments", child.Summary())
+				}
+				haveDefault++
+			}
 			missing = append(missing, strconv.Quote(child.Name))
 		}
 	}
-	if len(missing) == 0 {
+	if haveDefault > 1 {
+		return fmt.Errorf("more than one default command found under %s", node.Summary())
+	}
+	if len(missing) == 0 || haveDefault > 0 {
 		return nil
 	}