Shared *and* individual configuration in Sinatra.

app.rb

require 'sinatra'

configure :development do
   set :one, 1
end

configure :production do
   set :one, 11
end

configure :production, :development do
   set :three, 3
end

get "/" do
  haml <<STR
%p= "settings.one: #{settings.one.inspect}"
%p= "settings.three: #{settings.three.inspect}"
STR
end

# run `bundle install --binstubs --path vendor`
# and then
# `env RACK_ENV=development bundle exec ruby app.rb` and
# `env RACK_ENV=production bundle exec ruby app.rb`
# to see that you get different `one`s but the same `three`.