Add tileSize property

[a1gemmel]

It would be useful to be able to have tileSize as a TileJSON property to make it easier to use non-default sized raster tiles.

[vicapow]

It does seem like it would make more sense for this to be part of TileJSON though it is a part of the MapboxGL-style spec. https://www.mapbox.com/mapbox-gl-js/style-spec/#sources-raster-tileSize

[nigels-com]

+1 for this. All our tiles are 512x512 but we'd like to handle 256x256 for 3rd party sets.

[GretaCB]

Thanks for the ticket. Could you say a bit more about your usecase(s)? Also, would the proposed addition in #16 address some of these needs?