If you're interested in contributing, modifying, or otherwise messing around with this, here's a quick description of the dev setup.
I've developed this with Node v0.8.18. It should work on 0.10.x, but you may find otherwise (and please open a ticket when you do).
To get all dependencies:
npm install
The rest of the build is driven by Grunt. The Grunt CLI is included as a dev dependency, so you don't need it installed globally. I've added script aliases for the key Grunt tasks, so you can npm run foo instead of grunt foo for basic tasks.
Build the mapjack.user.js:
npm run build
(If you're following along, this would be grunt build if you have Grunt installed globally.)
Set up a watch to continuously rebuild mapjack.user.js:
npm run watch
(You guessed it, this is grunt watch for the global install.)
The resulting mapjack.user.js file is built in .grunt/mapjack/dist. The steps to load this as a user script depends on your browser extension.
With Tampermonkey you can @require the user script on your filesystem during development. For example, if the path to the build script is /path/to/mapjack/.grunt/mapjack/dist/mapjack.user.js then you could create a new script in Tampermonkey that looks like this:
// ==UserScript==
// @name mapjack
// @namespace http://tschaub.net/mapjack/
// @version 0.0.5
// @description Map based editing of GeoJSON files on GitHub.
// @match https://github.com/*/*/edit/*/*.geojson
// @grant unsafeWindow
// @require file:///path/to/mapjack/.grunt/mapjack/dist/mapjack.user.js
// ==/UserScript==
With the latest Tampermonkey, scripts that use the file:// scheme are not cached, so you should be able to reload after making edits and pull in your edits.