Tilejson fixups

[bdon]

@mikeocool

• This makes the serve command consistent with show --tilejson in taking a --public-url option for the base path. This is renamed from --public-hostname because it includes not just the hostname but also the protocol like https:// and possibly a non-root path like https://example.com/tiles.

• Also makes the above consistent where the TileJSON automatically adds the {z}/{x}/{y}.{ext} onto the end, inferring the tile type from the header, instead of having to present this yourself. @mikeocool any limitation in inferring this automatically?

• Finally, the TileJSON spec has some optional properties name, description, attribution, version and we omit those keys if empty instead of passing a null in TileJSON.

[mikeocool]

Late to the party on this one -- but LGTM! I worked the latest release into my tile build pipeline -- and it's great, exactly what i needed!