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.yml

+tasks:
+  - init: echo "Replace me with a build script for the project."
+    command: echo "Replace me with something that should run on every start, or just
+      remove me entirely."