Add option to not add empty lines between groups

**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
// 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';
```