Add option to not add empty lines between groups

[akshdeep-singh]

Is your feature request related to a problem? Please describe.
Using this, empty lines are always added between import groups, which I don't want.

Describe the solution you'd like
Add boolean option to enable/disable add empty line between groups.

Describe alternatives you've considered

Additional context
No Empty lines like below:

// Dart imports:
import 'dart:async';
import 'dart:io';
import 'dart:js';
// Flutter imports:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/painting.dart';
import 'package:flutter/physics.dart';
// Package imports:
import 'package:intl/intl.dart';
import 'package:mdi/mdi.dart';
import 'package:provider/provider.dart';
// Project imports:
import 'package:example_app/anotherFile2.dart';
import 'anotherFile.dart';

[vkammerer]

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