Support multiple GHC compilers at a time

Could be used with

```ruby
    chef.json.merge!({
      :haskell => {
        :multi => {
          :ghcs => {
            "7.6.3" => {
              :platform_version => "2013.2.0.0",
              :default => true
            },
            "7.4.2" => {
              :platform_version => "2012.2.0.0",
              :default => false
            },
            "7.0.4" => {
              :platform_version => "2011.4.0.0",
              :default => false
            }
          }
        }
      }
    })
```