Adds configuration options :fail_when and :notify_when

Example Usage:

``` ruby
I18n::Airbrake.configure do |config|
  config.fail_when = Proc.new do |handler|
    # Allow fallback to default plural rules
    handler.exception.is_a?(MissingTranslation) && handler.key.to_s != 'i18n.plural.rule'
  end

  config.notify_when = Proc.new do |handler|
    # Do not Notify in development or test
    %w( development test ).include?(Rails.env)
  end
end
```

Also documents configuration.