This adds a new utility called `martin-cp` that allows copying any
number of tiles from a single source (or a composite source) to an
mbtiles file.
`martin-cp` is a tool for generating tiles in bulk, and save retrieved
tiles into a new or an existing MBTiles file. It can be used to generate
tiles for a large area or multiple areas. If multiple areas overlap, it
will generate tiles only once. `martin-cp` supports the same
configuration file and CLI arguments as Martin server, so it can support
all sources and even combining sources.
