[WIP] Add id feature property to tiles

[gmaclennan]

GeoJSON supports the id field on Feature Objects. Vector Tile spec also supports id on features. Currently geojson-vt does not copy this id property into the vector tiles. This PR fixes that.

TODO:

• Update tests.

[gmaclennan]

ok @mourner this is ready. My only update to the tests was adding ids to the expected fixtures. I pretty-printed us-states-tiles.json first before updating so in that commit you can clearly see what has been changed. Since some fixtures don't have ids in the original GeoJSON and others do, that works as a test for whether ids are added correctly as needed (and no undefined ids are added)

[jfirebaugh]

@gmaclennan Can you make this two logical, standalone commits?

1. Pretty print tile JSON
2. Add id property

[gmaclennan]

See #60