feature: mountcabinet

I love the mountindex feature, (which I'm using in conjunction with samba.)

An excellent addition would be a mountcabinet feature which could be used on a top- or sub-cabinet.

I have a document which appears in several adjacent sub-cabinets, and it would be amazing to be able to mount the top-level cabinet and see that document in each subdirectory corresponding to each sub-cabinet.

It seems that this feature would also address the need of #596