Only import neovim module if necessary

- Delay importing of the neovim module until we check if --neovim
  as passed
Remove startup time for Neovim

- For Neovim we don't respect startuptime, instead we wait until the
  Neovim communication socket is created.
- To avoid a lock we stop waiting after a maximum time of 5s.
Update --startuptime usage msg for Neovim