tag: add passthrough for positional arguments

This new tag tells the parser to stop processing flags after the
positional argument is encountered.

This is particularly useful for subcommands that forward their arguments
to an external program, and makes it possible to implement commands
like `kubectl exec` or `docker run`.

Fixes #80.

context.go

@@ -324,6 +324,21 @@ func (c *Context) Reset() error {
 	})
 }
 
+func (c *Context) endParsing() {
+	args := []string{}
+	for {
+		token := c.scan.Pop()
+		if token.Type == EOLToken {
+			break
+		}
+		args = append(args, token.String())
+	}
+	// Note: tokens must be pushed in reverse order.
+	for i := range args {
+		c.scan.PushTyped(args[len(args)-1-i], PositionalArgumentToken)
+	}
+}
+
 func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 	positional := 0
 
@@ -349,18 +364,7 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 				// Indicates end of parsing. All remaining arguments are treated as positional arguments only.
 				case v == "--":
 					c.scan.Pop()
-					args := []string{}
-					for {
-						token = c.scan.Pop()
-						if token.Type == EOLToken {
-							break
-						}
-						args = append(args, token.String())
-					}
-					// Note: tokens must be pushed in reverse order.
-					for i := range args {
-						c.scan.PushTyped(args[len(args)-1-i], PositionalArgumentToken)
-					}
+					c.endParsing()
 
 				// Long flag.
 				case strings.HasPrefix(v, "--"):
@@ -413,6 +417,11 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 			// Ensure we've consumed all positional arguments.
 			if positional < len(node.Positional) {
 				arg := node.Positional[positional]
+
+				if arg.Passthrough {
+					c.endParsing()
+				}
+
 				err := arg.Parse(c.scan, c.getValue(arg))
 				if err != nil {
 					return err