Add ActorLimit adapter

[bkeepers]

This adds the ActorLimit adapter and configures it to be used by default.

101.times { |i| Flipper.enable :some_feature Flipper::Actor.new(i) }
# Error: Actor limit of 100 exceeded for feature some_feature

100 has been our recommended max for individual actors for a while, so I started with that default. It can easily be configured with:

Rails.application.configure do |config|
  config.flipper.actor_limit = 1000
  
  # or disable
  config.flipper.actor_limit = false
end

[jnunemaker]

Awesome!

ref #837