Separate validation into a distinct step.

This allows help to be called even when the parse trace is invalid.
Without this, the command-line would have to be valid in order to use
help at all, which defeats the purpose.

guesswidth.go

@@ -0,0 +1,9 @@
+// +build appengine !linux,!freebsd,!darwin,!dragonfly,!netbsd,!openbsd
+
+package kong
+
+import "io"
+
+func guessWidth(w io.Writer) int {
+	return 80
+}