[Gabriella439#58] Add intermediate representation

Problem: There is no way to reuse result of diff alghorithm because
program print it into stdout in human readable format in-place.

Solution: divide steps of finding diff between two derivations and
printing it out. Now result of `diff` is tree with result of derivation
comparison, that you can print than in stdout as was before, or use in
your program.

Author: s-and-witch

nix-diff.cabal

@@ -19,6 +19,9 @@ extra-source-files:  README.md
 
 executable nix-diff
   main-is:             Main.hs
+  other-modules:
+    Diff
+    Render.HumanReadable
   build-depends:       base                 >= 4.9      && < 5
                      , attoparsec           >= 0.13     && < 0.15
                      , bytestring           >= 0.9      && < 0.12