Allow to disable compression to help backups that allow for deduplication

[Davy Landman]

Currently the backups are all gziped, which makes perfect sense.

For our use case we are storing the backups on a share that gets daily backups (using borgbackup).

However, the deduplication (which works much like git's chunking algorithm) will not be able to recognize the 90% common parts between the backups, due to the fact that compression turns it into something close to entropy.

Are you open for a config option that disables the --gzip argument to mongodump?

[Sam Lin]

it is done here maxisam/mgob@d8b6959