Feature request: onclick and onevent can provide the javascript event details back to Shiny

[daattali]

The way it would look is you'd be able to provide a function (instead of an R expression) to onevent. I would still support expressions because it's easier to type than a function, and for backwards compatibility. If the function has an argument, the javascript event details could be passed as that argument. I'm pretty sure there will be problems serializing the javascript object because of cyclic values in the event.

Suggested in #91