I'm not sure these are really the right names, but the idea is that you
have some file you want to move, but you aren't real keen on typing out
a full destination path right now.  So you do something like:

    yank file_to_move_later
    # other stuff
    cd some_destination_directory
    unyank

The implementation should really be more general.  It would be cool if you
could stack files.  It _might_ be cool if it made hardlinks, for various
reasons, but I also suspect that complicates things.

Maybe these should be called `delete` and `put`?  Maybe the `unyank`
should copy rather than moving the file?  It feels like mostly that
isn't what I want, for various reasons.