Make trackGit ask if we want a pkgs.nix

[Chris Warburton]

If no pkgs.nix file is found, ask if we want one. If we do:

 - Ask whether we want nix-config.
 - Ask which nixpkgs revision to pin, with an example like '18.03'

We should copy across a template from ~/.templates.

We can look in ~/.templates for the requested nixpkgs revision and
sha256, and plug those in with a find/replace.

We can look in ~/.templates for a pkgs.nix template with nix-config or
one without. The one with should have a sentinel string we can replace
with the chosen nixpkgs version (perhaps with dots removed, e.g.
'nixpkgsREVISION' and we replace 'REVISION' with '1803').