[check-html-links] warn instead of error

[bennypowers]

Sometimes you know you're going to have broken links, and you're ok with it. check-html-links should add some config to warn instead of error. When set, it should exit with code 0

export interface CheckHtmlLinksCliOptions {
  printOnError: boolean;
  rootDir: string;
  ignoreLinkPatterns: string[] | null;
  continueOnError: boolean;
  // like
  warnOnBrokenLink: boolean;
  // or
  level: 'warn'|'error'
}

[bennypowers]

Further to this, I don't believe rocket cli currently lets users configure check-html-links if that's the case, it should be updated