Create a 'sync' command

[joshmedeski]

Per this conversation and other similar issues over the years does it make sense to create a "sync" to the "backup" command? It could potentially cause less confusion with what/how mackup is doing when you run it.

mackup backup

becomes

mackup sync

@lra thoughts? It's an easy update, and I'd suggest we keep mackup backup for those of us that are already used to that command.