Add support for alternate placetypes

[trescube]

WOF is incorporating a multi-valued altplacetype so records can be categorized in multiple ways. Add support for this. See whosonfirst-data/whosonfirst-data#948

[stepps00]

An existing record with a wof:placetype_alt property: https://whosonfirst.mapzen.com/spelunker/id/85921877/

Issue discussing logic around wof:placetype_alt: whosonfirst-data/whosonfirst-data#952

Right now, we are storing alternative placetypes in the wof:placetype_alt property for selected records. We plan on updating our tooling to also populate these alternative placetypes in the wof:hierarchy property, too.

The linked record above (Piedmont) is a locality, with a alternative placetype of neighbourhood, so it should be treated as both. The hierarchy will end up looking like:

    "wof:hierarchy":[
        {
            "continent_id":102191575,
            "country_id":85633793,
            "county_id":102086959,
            "locality_id":85921877,        <--
            "region_id":85688637,
            "neighbourhood_id":85921877    <--
        }
    ],