changes to knob for intro vignette

A few tweaks in anticipation of using knob in the intro vignette for htmlwidgets:

1) Made the widget_html function private to the package (it doesn't require exporting)

2) Changed the signature of knob to (a) not have a title (it wasn't being used); (b) use more explicit parameters rather than the ellipses (want to encourage developers to do this so R bindings are self-documenting rather than requiring users to rely on the docs of the JS library)

3) Added some support for dynamic sizing. This is still not perfect as knob wants to be a square (or close to it) and the Viewer pane and knitr figures aren't always square. Our solution is to force the width and height to Math.min(width, height).