Changelog for gray-extended

1.5.8 Simplified cabal file.

1.5.7 Simplified nix configuration.

1.5.6 Corrected copyright info.

1.5.5 Revamped to work with Nix + cabal-install

1.5.4 Revamped to work with Nix + Stack.

1.5.3 Upgrade to work with Stack 1.9.3

Unreleased changes