Allow commands to explicitly state if they do, or do not take arbitrary arguments

Check that arguments are in ValidArgs

If a command defined cmd.ValidArgs check that the argument is actually
in ValidArgs and fail if it is not.

bash_completions_test.go

@@ -117,6 +117,8 @@ func TestBashCompletions(t *testing.T) {
 	// check for filename extension flags
 	check(t, str, `flags_completion+=("_filedir")`)
 	// check for filename extension flags
+	check(t, str, `must_have_one_noun+=("three")`)
+	// check for filename extention flags
 	check(t, str, `flags_completion+=("__handle_filename_extension_flag json|yaml|yml")`)
 	// check for custom flags
 	check(t, str, `flags_completion+=("__complete_custom")`)