Add tile size to metadata

[rbrundritt]

In a future version, a tile size metadata option should be considered. Mapbox vector tiles are 512 square pixels in size, but raster tiles traditionally have been 256 square pixels, but in the last couple years, some have started moving to larger sizes. When it comes to storing the tiles in SQLite, the tile size makes no difference, but when rendering data, the tile size will often be needed. Adding an optional column to the metadata table for table size would address this. There is a similar request for the TileJSON spec here: mapbox/tilejson-spec#26