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.

model.go

@@ -236,6 +236,7 @@ type Value struct {
 	Set          bool   // Set to true when this value is set through some mechanism.
 	Format       string // Formatting directive, if applicable.
 	Position     int    // Position (for positional arguments).
+	Passthrough  bool   // Set to true to stop flag parsing when encountered.
 }
 
 // EnumMap returns a map of the enums in this value.