Add option for vim-keybindings in Ace editors

[vnijs]

Allow the user pass an argument to the exercise chunk for keybindings (e.g., vimKeyBinding = TRUE or FALSE) or set a user specific option that would activate:

editor.setKeyboardHandler('ace/keyboard/vim')

As implemented in shinyAce

https://github.com/trestletech/shinyAce/pull/9/commits/b089a4ec46e63d2fad421dbb662c2af72aeda667

ACE files to include: keybinding-vim.js (and perhaps ext-keybinding_menu.js)