add two functions withJS() and JSEvals() as well as a helper function in JS HTMLWidgets.evaluateStringMember()

so that we can mark certain elements of a list as JS code that can be evaluated later, e.g.

datatable(iris, options = list(pageLength = 10, render = JS('function(...) {...}')))

we can find the element `render` by JSEvals(substitute(options)), and HTMLWidgets.evaluateStringMember(options, 'render')