message: Support default option
diff: @@ -253,9 +253,9 @@ MatrixHandler.prototype._onAdminMessage = Promise.coroutine(function*(req, event
  summary: `Remove a previously stored NickServ password`,
  },
  "!feature": {
  - example: `!feature feature-name [true/false]`,
  - summary: `Enable or disable a feature for your account.` +
  - `Will display if true/false not given.`,
  + example: `!feature feature-name [true/false/default]`,
  + summary: `Enable, disable or default a feature's status for your account.` +
  + `Will display the current feature status if true/false/default not given.`,
  },
  "!quit": {
  example: `!quit`,
  @@ -726,14 +726,16 @@ MatrixHandler.prototype._onAdminMessage = Promise.coroutine(function*(req, event
  yield this.ircBridge.sendMatrixAction(adminRoom, botUser, notice, req);
  return;
  }
  - if (!["true", "false"].includes(args[1].toLowerCase())) {
  + if (!["true", "false", "default"].includes(args[1].toLowerCase())) {
  let notice = new MatrixAction("notice",
  "Parameter must be either true or false."
  );
  yield this.ircBridge.sendMatrixAction(adminRoom, botUser, notice, req);
  return;
  }
  - features[featureFlag] = args[1].toLowerCase() === "true";
  + features[featureFlag] = args[1] === "default" ? undefined :
  + args[1].toLowerCase() === "true";
  +
  yield this.ircBridge.getStore().storeUserFeatures(event.user_id, features);
  if (featureFlag === "mentions") {
  // We should invalidate caching for this user's channels.
  