Ability to define new tier of packages

[lukepighetti]

Would be nice if we could define a new tier of packages for import_sorter consider this case

// dart imports
import 'dart:async';

// flutter imports
import 'package:flutter/material.dart';

// package imports
import 'package:context_watch/context_watch.dart';

// internal shared package imports << add the ability to tag "level4: ptp_shared"
import 'package:ptp_shared/ui/ptp_app_bar.dart';
import 'package:ptp_shared/ui/show_app_bottom_sheet.dart';

// current project imports
import 'package:ptp/app/widgets/card_collection.dart';
import 'package:ptp/services/localization.dart';
import 'package:ptp/telemetry/analytics.dart';

[vkammerer]

This is something that could be handled by the next maintainer once #84 is resolved