Add support for type' for Buttons.

[vipentti]

This allows for setting the type of the button.

    Button.render ...
      [ Button.type' "button"
      ]
      [ ... ]

[debois]

Why not just type'?

[debois]

Made a few comments, but otherwise, merge at will.

Question of principle: Do we want the demo to exhibit or at least mention in text everything a component can do? Pro: It'd be great for users. Con: It'll be a ton of work for us, making we sure we have a demo of stuff like this, or at least mention it in text.