Default Instance Storage

[jnunemaker]

This adds a tidy little spot to configure the default flipper instance. This has long been a thing people have to think about to use flipper. Rarely do people use multiple flipper instances in the same app, and if they want to they still can, but at least now it is wrapped up for most people.

# these requires and the configure could sit in an initializer
require 'flipper'
require 'flipper/adapters/memory'

Flipper.configure do |config|
  config.default { Flipper.new(Flipper::Adapters::Memory.new) }
end

puts Flipper.enabled?(:search)
Flipper.enable(:search)
puts Flipper.enabled?(:search)
Flipper.disable(:search)

default is configured with a block. This makes it so it can either return a single instance no matter what or generate an instance safe for a thread, if necessary. A top level method Flipper.instance exists to memoize an instance of the default per thread.

module Flipper
  def instance
    Thread.current[:flipper_instance] ||= configuration.default
  end
end

Each thread should get its own flipper instance with this, which should keep things thread safe without much work. This seemed easier than synchronizing everywhere. Since Flipper::DSL instances are light, it seemed like a fine trade off. Let me know if you have any different thoughts.

All of the DSL methods (save group which is already top level in Flipper module) are now delegated to Flipper.instance. This makes it so instead of doing:

Flipper.instance.enabled?(:search)

You can do:

Flipper.enabled?(:search)

...which is shorter and easier to remember. The list of methods delegated at this time is here.

I'll let this sit for a day or so. Please drop any comments. If I've already merged, feel free to continue to drop comments and I'll follow up with subsequent PRs to address.

fixes #243
xref #261