Add ecto migration generator for new flags

[brain-geek]

While our feature flags are defined in code, after deployment to new environment someone needs to manually create those with current approach. That is sometimes prone to errors like creating feature-flag instead of Feature-flag or feature_flag, which creates confusion during testing.

To simplify this process, I have added an optional generator to generate new (newly disabled) flag during migration.

It's heavily based on ecto.gen.migration, and should help if you have multiple environments and want to deploy more feature flags.