Add internal ZIPing of individual tiles within tile pack MBTiles

[nvkelso]

As suggested in tangrams/tangram-es#1552 (comment) it may improve overall file size and is a valid MBTiles option.

[iandees]

We can definitely do this, but the best way to improve overall file size is to zip the entire mbtiles file, not individual tiles.

Compressing individual tiles just means there's slightly less data to read out of sqlite for each tile.