Add hdevtools linter for haskell

This adds support for the hdevtools haskell linter
https://github.com/hdevtools/hdevtools

The output for hdevtools is near identical to the ghc output so this
also extracts the ghc handler into the handle file and adds tests

* Add testing for previous major release of ghc

ale_linters/haskell/ghc.vim

@@ -1,10 +1,6 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: ghc for Haskell files
 
-if exists('g:loaded_ale_linters_haskell_ghc')
-    finish
-endif
-
 call ale#linter#Define('haskell', {
 \   'name': 'ghc',
 \   'output_stream': 'stderr',

ale_linters/haskell/hdevtools.vim

@@ -1,4 +1,4 @@
-" Author: w0rp <devw0rp@gmail.com>
+" Author: rob-b
 " Description: hdevtools for Haskell files
 
 call ale#linter#Define('haskell', {