Adding postal code format for the Netherlands

Very straightforward. Dutch postal codes consist of 4 digits followed by two upper-case alpha characters, optionally separated by a space. [Data from Google](http://i18napis.appspot.com/address/data/NL). 

__Proposed format__
`^\d{4}[ ]?[A-Z]{2}$`

__Examples__
- 1234 AB
- 2490 AA