Support Neovim commands in shell.vroomfaker

- Following #60
- Disable shell.vroomfaker command parsing when running Neovim
- When Vim calls the shell to run a command it wraps the command and
  redirects the output to a temporary file. When intercepting calls
  shell.vroomfaker removes the wrapping in order to match the command.
  However Neovim differs from Vim and passes the command to the shell
  unaltered.
- When running Neovim Vroom sets an environment variable VROOM_NEOVIM
  that causes shell.vroomfaker to disable the command parsing used for
  Vim