Adding haskell.nix based default and shell.nix (#141)

Co-authored-by: Ben Ford <ben@perurbis.com>

examples/demo_stars.hs

@@ -1,5 +1,12 @@
-#!/usr/bin/env stack
--- stack runghc --package reanimate
+#!/usr/bin/env cabal
+{- cabal:
+build-depends: base
+            , reanimate
+            , reanimate-svg
+            , vector
+            , random
+            , JuicyPixels
+-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ParallelListComp #-}
 module Main