Hide/Show code cells in HTML

I've added a tiny bit of javascript to the HTML converter.  Now, if you click on the input prompt, it will toggle the visibility of the corresponding code cell.  The code cells are also hidden by default, which makes it more suitable for presenting scientific results to collaborators.

It would be better to have a little "show/hide" text in very small font that would make it more obvious that this feature was available, but I'm not sure I know how to do that well.

This is both the first time I've written javascript and the first time I've really used github, so please be gentle.
Jon