haskell-vim-now supports installation on NixOS yet

Add ability to run haskell-vim-now installation within nix-shell

As described in 284 installation of haskell-vim-now it was impossible to install vim-now-haskell on NixOS or within nix-shell because lack of stack's --nix parameter.
For more information see Nix integration.

The PR 285 contains implementation of an additional parameter for the purpose to run stack in nix-shell.

The installer script takes --nix parameter and provides it to stack
Describe briefly and add screenshots/recordings if applicable.

Here the link to the changes in code and documentation: 974e41f

PS actually PR was reported in https://utopian.io/utopian-io/@palik/installation-of-haskell-vim-now-on-nixos but it took time until merging.