Compress manifests #7
Description
Staging the snapshot and timestamp manifests should optionally compress their dependant manifests, for example:
$ tree staged
staged
└── targets.json
$ tuf snapshot --compression=gzip
$ tree staged
staged
├── snapshot.json
├── targets.json
└── targets.json.gz
$ tuf timestamp --compression=gzip
$ tree staged
staged
├── snapshot.json
├── snapshot.json.gz
├── targets.json
├── targets.json.gz
└── timestamp.json