Use an initializer instead of an yaml file for configurations 

First of all, thanks for this awesome gem!

In this commit I've moved the `PrivatePub.load_config` call to an initalizer .rb file, generated by running `rails generate private_pub:install`. I think an intializer under your project tree is a better approach than an initializer in a `Rails::Engine`, because it enables you to perform several customizations in the way the settings are loaded. Think about a rails app that already defines its own config file and you want to avoid duplicates, and so on.

Well, it's just an idea. Even if you don't approve these changes, I would appreciate your comments.

Thanks.