Diminish interactive-haskell-mode correctly

`interactive-haskell-mode` is defined in haskell.el and without this change I'm getting `File mode specification error: (error "interactive-haskell-mode is not currently registered as a minor mode")` whenever I open a .hs file.
Merge pull request #240 from osener/patch-1

Diminish interactive-haskell-mode correctly
.hi files are now added to completion-ignored-extensions by haskell-mode
Enable eldoc-mode in haskell-mode
Update
Merge remote-tracking branch 'refs/remotes/purcell/master'