Allow mounting a layer at a subdirectory rather than at the root of merged directory

[WaterJuice]

Each merged source appears in the merged folders root. Ir would be very handy in some cases to be able to specify a "prefix" folder path for a particular source. I will illustrate what I'm trying to do with an example:

/mnt
    /folder1
        /extra
            /thing1
            /thing2
    /folder2
        /extra
            /thing1

Now if I merge these using
unionfs-fuse -o cow /mnt/folder1=RO:/mnt/folder2=RO:/mnt/rw=RW /mnt/merged
I get the following layout:

/mnt
    /merged
        /extra
            /thing1
            /thing2

Now let's say I only want the "thing1" folders in the merged thing. The layout I want is:

 /mnt
      /merged
           /extra
               /thing1

However there is no way to specify this.
If I try

 Unionfs-fuse -o cow /mnt/folder1=RO:/mnt/folder2/extra/thing1=RO:/mnt/rw=RW /mnt/merged

The folder2 "thing1" contents will be at the root level in the merged folder.

The syntax could be extended to allow something like

 Unionfs-fuse -o cow /mnt/folder1=RO:/mnt/folder2/extra/thing1=RO,prefix=extra/thing1:/mnt/rw=RW /mnt/merged

This is something I'm going to need and willing to implement in a fork, but thought I would check here first to see if it could be added to the official unionfs-fuse

[rarensu]

I usually do this by having multiple instances of unionfs-fuse running. I have a feeling it's not optimal, but it works great. they don't seem to mind if you start them out of order, either.