suppress warnings for using Params

[Etomyutikos]

When setting Params in gnorm.toml, gnorm gen prints a warning. This warning comes from BurntSushi/toml's MetaData.Undecoded(): https://github.com/gnormal/gnorm/blob/master/cli/parse.go#L38-L41

The warning should be suppressed because Params is designed to be a bag of whatever.