Ignore files generated by cabal sandbox

https://github.com/haskell/cabal/wiki/Package-Environments#sandbox-command

Haskell.gitignore

@@ -6,3 +6,6 @@ cabal-dev
 *.chs.h
 .virthualenv
 .hsenv
+.cabal-sandbox/
+cabal.sandbox.config
+cabal.config