get postal cities countries from pelias-config

[NairolfL]

👋 I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

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Here's the reason for this change 🚀

• Configure countries to use for postal city lookup #305

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Here's what actually got changed 👏

• Added the possibility to configure the postal cities countries via pelias-config

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Running the tests should work as it used to as the default countries are the same.
Configuring imports.adminLookup.postalCitiesCountries (array) should load the configured countries instead of default ones

[missinglink]

Hi @NairolfL, what's the status of this, do you want to bring it out of draft or should we close it?
I feel like this is a nice thing to have, did you find it useful?