Add secret loading railtie

[gmcgibbon]

I think there's value in adding a railtie that automatically merges ejson secrets into Rails' secrets. Otherwise, maybe we can do this in an ejson-rails style gem?

[burke]

I like this idea and the implementation seems sensible but I think I would prefer it as a separate ejson-rails gem.

[gmcgibbon]

Sounds good 👍