Mapping: keep/forward any <Plug>\w+CR mappings

This changes the map forwarding, which has been used for
<Plug>delimitMateCR, to be more generic.

I plan to setup a <Plug>-mapping for cursorcross [1], and endwise should
keep it.

You might want to make it act on any "<Plug>..." mapping. I have kept
the `CR` suffix as some kind of safety net.

1: https://github.com/mtth/cursorcross.vim