Add <Plug> mapping

Author: gpanders

README.md

@@ -54,7 +54,7 @@ yourself by adding the following to the file
 `~/.vim/after/ftplugin/markdown.vim` (create it if it does not yet exist):
 
 ```vim
-nnoremap <buffer> Z! :<C-U>EvalBlock<CR>
+nmap <buffer> Z! <Plug>(medieval-eval)
 ```
 
 ## Limitations

ftplugin/markdown/medieval.vim

@@ -5,4 +5,6 @@ command! -bang -buffer -nargs=? EvalBlock
             \   call medieval#eval(<q-args>) |
             \ endif
 
+nnoremap <silent> <Plug>(medieval-eval) :<C-U>call medieval#eval()<CR>
+
 let b:undo_ftplugin = get(b:, 'undo_ftplugin', '') . '|delc EvalBlock'