Add mapping feature

[backdround]

I have a remapped 'u' key, so i tried to map undo keys to another by #58:

if empty(mapcheck("<Plug>(RepeatUndo)"))
  nnoremap <Plug>(RepeatUndo) u
endif

It works, but sometimes vim reset my 'u' key to undo. And I dont know why

Here i divided into two files and add option to disable default mappings

[marcoaaguiar]

Merging this would be appreciated!

[tpope]

I merged another PR that solves this with hasmapto().