Add Strict adapter to ensure feature exists on get

[bkeepers]

First pass at adding a strict adapter to ensure only features that exist can be called.

Flipper.configure do |config|
  config.adapter { Flipper::Adapters::Strict.new(Flipper::Adapters::ActiveRecord.new) }
end

Flipper.enabled?(:featuere_name) # raises Flipper::Adapters::Strict::NotFound

Flipper.add(:feature_name)

Flipper.enabled?(:feature_name) # => false

In a follow up PR I'm hoping to use this by default in Rails apps in development/test.

[jnunemaker]

Looks good. Left a couple questions. I'd think get_multi should be in so there is a raise/warning for preloading the wrong feature name.

[garrettdimon]

Are you envisioning that this would be used like in the example or would it also have some level of configuration? i.e. Is the configuration happening purely from how the initializer is called?

[bkeepers]

Are you envisioning that this would be used like in the example or would it also have some level of configuration? i.e. Is the configuration happening purely from how the initializer is called?

Still in progress, but this is what I am imagining: #763