ConfigOption#has_key? to identify key presence.

Example usage:

```ruby
ExampleConfig.has_key? :some_key #=> true
ExampleConfig.has_key? :nonexistant #=> false

ExampleConfig.some_key.has_key? :other_key #=> true
ExampleConfig.some_key.has_key? :nonexistant #=> false
```

Fixes #4