Add ability to export collection metadata when dumping to stdout

As it says in the docs, if you use mongodump with "--out -" to dump to stdout, it won't write the files <dbname>.metadata.json. That means, if you want to pipe the dump directly to another tool (gzip, ssh, ...), there is currently no easy way to store the metadata as well. You would have to use manual MongoDB queries to construct these json files (if I don't miss anything).

I think an option "--metadata-only" would solve this issue. This solution might not be the best, but probably the easiest.