Added <Plug>SchleppDup and <SID>SchleppDup.

No-arg dup mappings. Rely on schlepp options for direction.

I was wondering why the following vmap, which I added to my vimrc, wasn't working:

    vmap ,d <Plug>SchleppDup

I dug into the plugin code and found that <Plug>SchleppDup wasn't actually defined. So I added that and now my vmap works.