Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitHub and GitLab that enables Dev-Environments-As-Code.
This makes it easy for anyone to get a ready-to-code workspace for any branch,
issue or pull request almost instantly with a single click.

Author: AJ Kerrigan

.gitpod.Dockerfile

@@ -0,0 +1 @@
+FROM python:3

.gitpod.yml

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+image:
+  file: .gitpod.Dockerfile
+
+tasks:
+  - init: pip install -r ./dev/requirements-dev.txt && pip install -e .

.theia/settings.json

@@ -0,0 +1,8 @@
+{
+    "python.testing.pytestArgs": [
+        "visidata"
+    ],
+    "python.testing.unittestEnabled": false,
+    "python.testing.nosetestsEnabled": false,
+    "python.testing.pytestEnabled": true
+}

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/saulpw/visidata)
+
 # VisiData v2-5dev [![CircleCI](https://circleci.com/gh/saulpw/visidata/tree/develop.svg?style=svg)](https://circleci.com/gh/saulpw/visidata/tree/develop)
 
 A terminal interface for exploring and arranging tabular data.