Allow setting of feature state defaults that are used when the adapter is down

Motivation: when using the redis adapter we don't want redis down-ness to cause application down-ness . Obviously we could wrap & rescue all our `.enabled?` calls (or patch the adapter etc) but it seemed that this might be something other people find useful

Anyway, thought it'd be worth getting your thoughts on this direction

This is a initial stab at adding this functionality:
```ruby
flipper = Flipper.new adapter
flipper.feature_defaults = { stats: true }

flipper[:stats].enabled?  # will return true if adapter.get raises
flipper[:search].enabled? # will raise if adapter.get raises
```

Another direction would be to add a something that wraps a regular adapter. Something like:
```ruby
redis_adapter = Fliper::Adapters::Redis.new(...)
adapter = Flipper::RescueAndDefault.new(redis_adapter,
  defaults: { stats: true })
```

Alternatively perhaps this is too use case specific and shouldn't be included in the core gem?