Add gitpod config

this commit adds support for Gitpod.io, a free automated
dev environment that makes contributing and generally working on GitHub
projects much easier. It allows anyone to start a ready-to-code dev
environment for any branch, issue and pull request with a single click.

.gitpod.Dockerfile

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+FROM gitpod/workspace-full
+                    
+USER gitpod
+
+# Install custom tools, runtime, etc. using apt-get
+# For example, the command below would install "bastet" - a command line tetris clone:
+#
+# RUN sudo apt-get -q update && #     sudo apt-get install -yq bastet && #     sudo rm -rf /var/lib/apt/lists/*
+#
+# More information: https://www.gitpod.io/docs/config-docker/

.gitpod.yml

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+tasks:
+  - init: make
+image:
+  file: .gitpod.Dockerfile

README.md

@@ -1,3 +1,5 @@
+[![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/ytdl-org/youtube-dl) 
+
 [![Build Status](https://travis-ci.org/ytdl-org/youtube-dl.svg?branch=master)](https://travis-ci.org/ytdl-org/youtube-dl)
 
 youtube-dl - download videos from youtube.com or other video platforms