Added `getItem`

`getItem` retrieves an item using provided `ItemId`. That function might
be useful to fetch an item when its type is not known beforehand.

```haskell
*Web.HackerNews> hackerNews $ getItem $ ItemId 12345
Just (ItemStory (Story {storyBy = "bootload", 04-12 21:06:41 UTC, storyTitle = "Distributed file storage: MogileFS", storyType = "story", storyUrl = "http://www.uncov.com/2007/4/4/distributed-file-storage-with-mogilefs", storyDeleted = False}))
```