Add 'dump's parameter — e.g., $ mb-util world.mbtiles dumps

[roblabs]

mb-util is very useful, and I found myself extracting tiles to disk just to peek at the metadata.json.

This pull request adds a new parameter that uses json.dumps to dump the contents of the metadata to stdout.

usage

mb-util world.mbtiles dumps

[tmcw]

:+1: thanks!