Enable messages from Neovim to vimcom.plus.
The Vim-R-plugin now can send messages to R if running in Neovim.
Current known bugs affecting Neovim:
  - Omnicompletion and completion of function arguments don't display
    the pop up menu with the list of completions.
  - Multiline messages don't display completely. Only the last line is
    shown.
The vimcom.plus package still cannot send messages to Neovim. It can
only reply to messages received from Neovim.