Add Heroku button for one-click deploys

[mcmillan]

Because I'm fundamentally lazy, when I stumble across projects that are easy to deploy on Heroku it's awesome to have a one-click Heroku Button. This automatically sets up the app based on the contents of app.json in the project root when clicked.

You can see how this might work by visiting this URL – it basically asks you for the required environment variable and then goes ahead and sets up a shiny new Heroku app for you.

[CharlieHess]

That's awesome, thanks @mcmillan!