facilitate calls to result.continent.code

`continent` is one type of `named resource` returned by a given result, except it carries a `code` identifier instead of an `iso_code` identifier.

```` rb
result.continent
=> #<MaxMindDB::Result::NamedLocation:0x007f935f919d18 @raw={"code"=>"NA", "geoname_id"=>6255149, "names"=>{"de"=>"Nordamerika", "en"=>"North America", "es"=>"Norteamérica", "fr"=>"Amérique du Nord", "ja"=>"北アメリカ", "pt-BR"=>"América do Norte", "ru"=>"Северная Америка", "zh-CN"=>"北美洲"}}>
````

```` rb
result.continent.code
=> NoMethodError: undefined method `code' for #<MaxMindDB::Result::NamedLocation:0x007fc41c953db8>
````

this pull request adds `code` to the list of named methods for a `named resource`. 

```` rb
result.continent.code
=> "NA"
````