UTFGrid: Specify what field to use for an object's key

[Mr0grog]

It would be nice to specify what field to use as an object's key in a UTFGrid's data object.

For example, if we had the following GeoJSON object:

{
  "type": "feature",
  "geometry": { ... },
  "properties": {
    "name": "City Hall",
    "building_id": 15,
    ...
  }
}

And we specified that building_id should be used, we'd get the following UTFGrid:

{
  "grid": [" !",
           "  "],
  "keys": ["", "15"],
  "data": {
    "15": {"name": "City Hall", "building_id": 15, ... }
  }
}

Or if we specified name:

{
  "grid": [" !",
           "  "],
  "keys": ["", "City Hall"],
  "data": {
    "City Hall": {"name": "City Hall", "building_id": 15, ... }
  }
}

This is probably not a huge deal, but I can imagine cases where it might be useful. Not top priority for sure, though.