Synchronize command line opts with function opts.

This loop in `write-good.js` was silently failing by returning
`opts[X] == undefined` instead of `false` for some options,
making the command line flags behave unexpectedly.

  Object.keys(checks).forEach(function (checkName) {
    if (opts[checkName] !== false) {
      suggestions = suggestions.concat(checks[checkName].fn(text).
                          map(reasonable(checks[checkName].explanation)));
    }
  });