cabal-install: Add bash completion

The bash completion shipped with cabal-install package is now installed
to bash_completion.d directory.

Closes Homebrew/homebrew#8261.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/cabal-install.rb

@@ -10,5 +10,9 @@ class CabalInstall < Formula
   def install
     ENV['PREFIX'] = "#{prefix}"
     system "sh bootstrap.sh"
+
+    cellar_etc = prefix + 'etc'
+    bash_completion_d = cellar_etc + 'bash_completion.d'
+    bash_completion_d.install 'bash-completion/cabal'
   end
 end