maps: forward existing CR maps / use <Plug>

This is based on how endwise [1] handles it, but requires a patch/pull
request to endwise to work correctly [2].

Ref: https://github.com/mtth/cursorcross.vim/issues/1

1: https://github.com/tpope/vim-endwise
2: https://github.com/tpope/vim-endwise/pull/47