add heroku support

[yoshiori]

Add support for HEROKU's dyno-metadata.
https://devcenter.heroku.com/articles/dyno-metadata

[sorah]

Adding support for a specific vendor is too adhoc, I may accept the patch if it takes an environment variable name instead of something hard-coded.

However, to return a specific environment variable can be achieved by the following code, so I don't think you need a revision_plate:

# config.ru
map '/site/sha' do
  run lambda {|env| [200, {"Content-Type" => "text/plain"}, ["#{ENV["HEROKU_SLUG_COMMIT"}}\n"]] }
end

# config/routes.rb
Rails.application.routes.draw do |r|
  r.get '/site/sha' => lambda {|env| [200, {"Content-Type" => "text/plain"}, ["#{ENV["HEROKU_SLUG_COMMIT"}}\n"]] }
end

But I understand if an app runs on multiple platforms, support loading from environment variables may help.

So in conclusion, I wish the interfaces like:

run RevisionPlate::App, nil, revision_proc: -> { ENV["HEROKU_SLUG_COMMIT"] }

and revision_proc should take precedence than a file.