Additional Link click option

This PR deals with a couple deficiencies in the `Link` code, some of which were called out on other issues:

* Didn't take an options hash properly, so you couldn't style your links. Boo!
* Didn't support `Link#click` which let you override the click-block after the link was created. Part of #528 
* Didn't support pass a string in the `:click` key of the options, which was relied on by the original `class-book.rb` example. #588 

One subtle bit of the change was that `Shoes::Swt::Link` needed to pass a wrapping block to the `clickable` call that would delegate back to the link itself, otherwise we capture the block at `initialize` and subsequent calls to `click` would be ignored.

These changes don't entirely fix those other related issues, but it's a good step toward them.