Improve extensibility/plugability

[Timoses]

First draft..

Callbacks:

• s:print(name, itemsOrMenus)
  • prints the menu, each row is printed using formatEntry (in default implementation)
• s:formatEntry(rowNum, entry)
• s:select(choices)

Use cases:

• custom numbering/lettering of menu entries (idea from https://github.com/Timoses/vim-venu/issues/7)
  • simply add a key shortcut to the items or menus when creating them
  • use formatEntry to print that shortcut
  • use select to return the selection

[ozars]

Looks good!