registerInputHandler support, redux

[alandipert]

This is a second attempt at adding support for shiny::registerInputHandler(). In my first attempt, I didn't understand the purpose of getType(). Now, I understand that an input's type is a namespace distinct from the name argument of Shiny.inputBindings.register() and with a different extent. Using an input's name as its type, as I did in #26, made no sense.

An input's name is used to alter the priority of an input in the event of a DOM conflict, and there is a 1-1 correspondence between an input and its name.

An input's type, on the other hand, has a different purpose. In R, shiny::registerInputHandler() is used to associate a type with an input handler that will coerce an input's value with a a matching type from a JSON representation to some alternate R representation.

So, every input has one and only one name, and also (optionally) a type, but multiple inputs may share a type.

To implement this, I added a new property to the options argument of reactR.reactShinyInput(): type. type may be a function, used as the input's getType() implementation. For expedience, type may also be false or a type name string, which is probably the most common use case.

I updated https://github.com/react-R/colorpicker-example to demonstrate usage of the functionality in this PR.

[timelyportfolio]

@alandipert great work, thanks! This will be very nice to have for custom situations, which I think will be more common with reactR inputs.