More-Safely Set Config Values

Do not overwrite existing config entries. This will prevent accidentally
breaking your app because you happened to name a YAML file the same as a
possibly unrelated config value.
Better Handle the Behavior of Rails config object

It behaves slightly differently than OpenStruct:

```ruby
> Rails.application.config.foo.nil?
=> NoMethodError: undefined method `foo' for #<Rails::Application::Configuration:0x007ff694237d18>
> Rails.application.config.respond_to? :foo
=> false
> Rails.application.config.foo = nil
=> nil
> Rails.application.config.foo.nil?
=> true
> Rails.application.config.respond_to? :foo
=> true

> config = OpenStruct.new
> config.foo.nil?
=> true
> config.respond_to? :foo
=> true
```
Merge pull request #4 from sportngin/safety-dance

More-Safely Set Config Values