Allow syncing single notebook

[amberin]

Same as orgzly/orgzly-android#653.

Auto-sync would be much more attractive to me if sync could be made faster. One way to make it faster is to allow syncing a single notebook.

For Git repos (and possibly others? Idk) changes on the remote side obviously must be handled. A remote change in a different notebook should probably lead to that notebook being synced as well, which is a potentially confusing UX. But such changes should typically be rare for someone using auto-sync.

[amberin]

This is highly related to #1, since that issue would entail major changes to the current syncing logic.