Sync for cloud/local folders

[pjbull]

We don't currently support any sync API. Even with upload_to and download_from, we will never delete files.

I'm not sure that we need/want to do anything about this, but wanted to capture some thoughts as I considered it. There are two options that I could see being reasonable:

• Explicitly document sync as a non-goal - Add to documentation and link to tools that sync cloud/local
• Add a sync API - we can probably do this just with copytree + removing files that don't exist on the source. That said, I don't think we want to be a sync tool so we should document this is for convenience, not performance.

[analog-cbarber]

You definitely should add sync, otherwise you force users to have to get it from somewhere else. I am pretty sure that at least S3 and Azure both support it in their client API.

[ypicard]

AWS S3 supports it with their CLI, but not with the boto3 SDK, which makes it a key feature that I would love to see happen in this library.

[pjbull]

Duplicate of #14. Consolidating.