New CLI feature: pmtiles merge

The pmtiles CLI now includes the highly requested merge command for combining multiple tilesets into a single archive. As an example use case: the Mapterhorn project distributes several archives: planet.pmtiles for the full planet terrain mosaic, from zoom levels 0 to 12. 6-20-28.pmtiles , 6-21-29.pmtiles , etc for zoom 13 and above in specific areas.