Add cabal-ghc linter

cabal-ghc calls ghc via cabal exec and so ghc has access to packages
in cabal sandboxes for example

ale_linters/haskell/cabal_ghc.vim

@@ -0,0 +1,18 @@
+" Author: Eric Wolf <ericwolf42@gmail.com>
+" Description: ghc for Haskell files called with cabal exec
+
+call ale#Set('haskell_cabal_ghc_options', '-fno-code -v0')
+
+function! ale_linters#haskell#cabal_ghc#GetCommand(buffer) abort
+    return 'cabal exec -- ghc '
+    \   . ale#Var(a:buffer, 'haskell_cabal_ghc_options')
+    \   . ' %t'
+endfunction
+
+call ale#linter#Define('haskell', {
+\   'name': 'cabal-ghc',
+\   'output_stream': 'stderr',
+\   'executable': 'cabal',
+\   'command_callback': 'ale_linters#haskell#cabal_ghc#GetCommand',
+\   'callback': 'ale#handlers#haskell#HandleGHCFormat',
+\})