OSM-POI: Introduce subcategories

[mattigrthr]

The tags belonging to one of the high-level categories can be further clustered into subcategories. These can then be used as filters in queries (see #16).

The example structure based on kuwala-pipelines/osm-poi/resources/categories.json would look like this (naming for subcategories: highLevelCategory_subcategory):

{
  "sport": {
    "category": "sport",
    "tags": [
      …
    ],
    "subcategories": [
      {
        "category": "sport_fighting",
        "tags": [
           …
        ]
      },
      …
    ]
  },
  …
}