* 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
After some thinking, it seems `mbtiles summary` (aliased as `mbtiles
info`) would be a bit better than stats. I renamed and adjusted
documentation, consolidating it in one doc page.
Other changes:
* use file system's file size, reporting 'unknown' if needed
* report page count
* moved bbox computation into a separate function
* inlined a number of things for readability
