* BREAKING: `martin-cp` will now set `format=pbf` instead of `mvt`. This
is what QGIS and possibly others expect, and this is what tools like
tilelive generates.
* `martin-cp` sets `minzoom` and `maxzoom` metadata values based on the
zoom parameters
* Add `mbtiles meta-update` command to refresh zoom levels based on the
present tiles.
Partially addresses items in #1081
