tweaks to scaffoldWidget function

- Forward the width and height parameters to createWidget
- Provide default width and height for Shiny output binding
- Use idiomatic 'x' for transmitting data from R to JS
- Don't use ... as an input argument (want to encourage explicit parameters for self-documenting bindings)
- include instance variable in JS binding
- add some more whitespace to JS binding
- don't import htmltools (not required unless generating custom widget HTML)