Update to support node-mapnik _render_grid() data option.
Update to support node-mapnik _render_grid() data option.

tomhughes/tilelive.js,topwood/tilelive,mapbox/tilelive.js,mapbox/tilelive,paulovieira/tilelive,nyurik/tilelive.js,Norkart/tilelive.js,kyroskoh/tilelive,gravitystorm/tilelive.js