Heroku button now supports URL parameters to override env

Change effective on 02 September 2015

You can now pass URL params in Heroku Button URLs to override env vars found in app.json. This example will cause Heroku to fill in testdomain for the SLACK_SUBDOMAIN env var:

 https://heroku.com/deploy?template=https://github.com/rauchg/slackin/tree/0.5.1&env[SLACK_SUBDOMAIN]=testdomain

Details are available on Dev Center