`get-objects/put-objects` `--skip` and `--skip-hash` options

[simonw]

Idea:

• --skip to skip downloading a file if it already exists with the same filename
• --skip-hash to skip downloading a file if it already exists AND the MD5 hash has not changed (more expensive as needs to calculate the local hash)

Originally posted by @simonw in #78 (comment)

[simonw]

Can add this for put-objects too:

• s3-credentials put-objects command #68