Add sync command

## Usecase

I'm automatically downloading files on a headless Linux server for one of our team having near to no technical knowledge from a SFTP host and providing those files (filtered) to that team in a Dropbox folder.
## Current way to use `dbxcli`
- Use `find` to list all files
- For each file execute `dbxcli put ${file} ${target}`

This has multiple disadvantages:
- DNS blocking using `block-api.g1.dropbox.com`
- Requests for files not required to sync
## Expected solution
- Call `dbxcli sync ${source_folder} ${target_folder}`

Advantages:
- Files are synced when required but not every time
- Fewer DNS requests / connections to establish
- More efficient sync (better for local system / better for Dropbox servers)
- Sync can be executed parallel instead of sequentially saving even more time