Add the warning for Options fields that published

[kazhuravlev]

options-gen should print the warning about field S3Endpoint (because ayone can change this option without constructor) in that case:

type Options struct {
  S3Endpoint string `option:"mandatory"`
}

• Print warning about public field in Options struct.
• Allow to disable warning by specify an argument like //go:generate options-gen .... --mute-warnings. By default it is enabled.
• Previous flag should disable warnings about deprecated option flags (required, not-empty).

[Antonboom]

Maybe by default to issue not a warning, but an error?
Thus to work like a linter.

[kazhuravlev]

@Antonboom It sounds interesting, but I will create a separate issue for this idea. I will add it in the next major version. This will break compatibility right now.