Added support for links with the target attribute

I added a small feature that allows a target attribute on link tags.

An example:

```haskell
main : Element                                                                    
main = flow down                                                                  
        [ linkWithTarget "http://google.com" "_blank" <| plainText "Link With Target"
        , link "http://google.com" <| plainText "Link without target"             
        ]  
```

The top link when clicked will open a new tab or window, depending on the browser to google, while the bottom one will change the current page to google.