Added TKSelect. 

Like TKToggle, but for a list of child element values of any length.
      ...
      Add: TKSelect to TangleKit.js
      Add: .TKSelect class to TangleKit.css
      ...

For a working example, see [this post on my blog](http://jthompson.ca/2013/03/playing-with-explorable-explanations/).