Add Downloader

[bezzad]

Project: https://github.com/bezzad/Downloader
PR: #729

1. Have your repository in Github or any source control been created at least 3 months?
   Yes
   First commit: 24 Mar 2020 And started with .Net Standard 2
   Latest commit: 21 Nov 2020
   170 commits until now.

2. The reason you think your library/framework/tooling/link is awesome?
   Downloader is a modern, fluent, asynchronous, testable and portable library for .NET. This is a multipart downloader with asynchronous progress events. This library written in .Net Standard 2 and you can add that in your .Net Core or .Net Framework projects.

3. Have your PR follows up with alphabet-ordering?
   Yes

4. Is it .NET Core or .NET Standard?
   Yes

5. Have you read through the README or issues list to make sure that your PR is not duplicated to any others?
   Yes