Add setting for ignoring empty directories

[Dekkonot]

Empty directories are currently interpreted by Rojo as being an empty Folder. This is fine, except that source control like git often leaves behind empty directories, because they manage files and not directories. This means that Rojo can often end up syncing in a lot of extra stuff that you don't need.

I propose we add a setting for ignoring empty directories, so that we don't have to deal with this.