Add ability to make the local XML repository optional

It would be handy to have the ability to edit remote wiki pages over REST without having a local XML repository. This happens when the wiki pages are not hosted in a Git repository beside their mere existence in the wiki instance, for instance when doing development before the initial commit, or for XWiki Cloud instances which are not meant to have a Git repository.

The initial set of synchronized pages could be in that case configurable from the command line, by specifying one or several target spaces, eg "–spaces Content XWiki.MyProject" etc.