Give Feature Flags default setting

This MR modifies the featureflag package to turn feature flags into a struct so a consistent default setting can be used. This allows us to define certain feature flags as default on.

It also modifies the ruby code's feature_flags.rb to allow passing in of a default value. There is a bit of inconsistency in that the ruby feature flags don't have a default setting. This is because we don't want to keep two lists of constants and have to keep them in sync.