Experiment with TIF output

Per voice, in the [raster branch](https://github.com/mapbox/tilelive-bridge/tree/raster) tilelive-bridge is currently hardcoded to export 1024x1024 webp tiles for any sources it detects to contain raster data.

To treat rasters more like tiled-performant-raw-data that you can manipulate on the fly/iterate on quickly when styling to get the final output product you want we can experiment with TIF as output format.

I have _never_ used this option in mapnik. Note we won't need any geotif tags as far as I can see (the geo is more or less implied by the tiling/zxy) but I am curious how this output format works in mapnik.

https://github.com/mapnik/mapnik/wiki/Image-IO#tiff-output-options

cc @springmeyer