Added .stack-work/ directory to Haskell .gitignore.

.stack-work/ is a work directory used by the `stack` build tool.

The `stack` build tool has recently been gaining a lot of traction in
the Haskell community.  It is very similar to the `cabal` build tool,
which also has entries in this Haskell .gitignore file.

Haskell.gitignore

@@ -13,3 +13,4 @@ cabal.sandbox.config
 *.prof
 *.aux
 *.hp
+.stack-work/