nextcloudcmd with list of folders to sync

`nextcloudcmd` has the `--unsyncedfolders` to pass the list of folders to **exclude** from syncing.

I miss a way to pass a list of folders to sync, because I need to sync just a few folders, not my entire cloud.

Currently, if I need to sync 10 folders I need to run 10 different `nextcloudcmd`. This feature would let me run 1 `nextcloudcmd` to sync all these 10 folders.

Or, even better, a unified file format to define what to sync and what to not sync. For example:

```
+/folder1
+/folder2
-/folder2/hugesubfolder
+/folder3
```

Would sync only `/folder1`, `/folder2` and `/folder3` and would skip `/folder2/hugesubfolder`.

Of course this is just an example and suggestion. This format might be weak and not too generic.

Start at the nextcloudcmd CLI handling for --unsyncedfolders and trace how its exclusion list reaches sync selection. The issue leaves open whether to add an include-folder list or a unified include/exclude format, so first establish the accepted syntax and precedence; done means one invocation can select multiple folders while excluding a nested folder.

Written by the indexing model from the issue text.