Include Rails integration in `dotenv` gem and refactor

[bkeepers]

This refactors the Rails integration, and includes it in the dotenv gem by default.

• Renames Dotenv::Railtie => Dotenv::Rails
• Adds Dotenv::Rails.files, which can be mutated to change which files are loaded
• Adds Dotenv::Rails.overwrite = true to overwrite existing ENV variables

I thought about deprecating the dotenv-rails gem, but for now I can just keep releasing it as an empty gem with a dependency on dotenv.