Add a `sync_folder` method

[pjbull]

We currently think it is a non-goal to manage folder syncing behind the scenes, but we might want to explicitly support it as a method.

For example, something like:

# I have a local folder that has many changes in it, including file deletions
local_changes = "/work_in_progress"

# copies cloud versions down to disk, but skips dl'ing where possible; deletes extra files locally
S3Path("s3://bucket/project_storage").sync_down(local_changes)

# copies local versions up to cloud, but skips ul'ing where possible; deletes extra files on the cloud
S3Path("s3://bucket/project_storage").sync_up(local_changes)

[ypicard]

This is the last missing piece that will make me migrate to this library. I would really appreciate this being a thing indeed.

[pjbull]

Additional discussion in #134.