Allow user to whitelist postalcode countries

[trescube]

The whosonfirst importer currently forces a user looking to import postalcodes to download and import postal codes from literally every single one of the 213 countries. That seems a bit excessive if, say, they're only interesting in importing Germany postal codes. Perhaps the importer can look for an array in pelias config of whitelisted country ISO2's to import. My personal use case is that I'm just trying to run a test import and I'm hacking the code a bit to just run a single country.

[orangejulius]

As of #487 and the https://github.com/pelias/whosonfirst/releases/tag/v4.0.0 release, we support countryCode as config parameter, and postal codes are only downloaded on a per-country basis! 🎉