ignore HTF test framework artifact directory

See http://hackage.haskell.org/package/HTF-0.13.1.0/docs/Test-Framework-CmdlineOptions.html (opts_historyFile)

Haskell.gitignore

@@ -17,3 +17,4 @@ cabal.sandbox.config
 *.eventlog
 .stack-work/
 cabal.project.local
+.HTF/