Easy React-based input bindings

[alandipert]

input binding support

This adds a new scaffolding function, scaffoldReactShinyInput, for scaffolding new inputs based on React. It also adds:

1. A new JavaScript function, reactR.reactShinyInput, for registering React components as Shiny inputs
2. A new R function, createReactShinyInput, for constructing inputs with custom configuration and a default value on the R side.
3. An additional tutorial vignette that demonstrates wrapping react-color with a custom input

In addition to the new scaffold function, existing scaffold code was extensively reworked. Scaffold functionality common between the widget and input scaffold functions were factored into helper functions.

[alandipert]

@timelyportfolio I think this is in good enough shape for you to play around with now. Thanks in advance for any feedback!

I hope to rope in @jcheng5 for a code review this week too.

I'll collect feedback from you both and then let you know when things are in a merge-worthy state.

[timelyportfolio]

@alandipert I got a chance to walk through the vignette from scratch, and everything worked nicely. I will try to walk through with a completely new and different component now.

I think we should add app.R' 'package.json' 'webpack.config.js' 'yarn.lock' to .Rbuildignore, so that the package is as near CRAN-worthy as possible and we adhere to those standards. If you agree, I'm happy to submit pull or feel free to change.

[alandipert]

@timelyportfolio thanks as always for the close review. I'm happy to make these changes. I got sidetracked this week helping with a Shiny release but I should be able to work on this tomorrow.

I'll also have a very small change I'll push soon that restricts the name argument accepted by the scaffolding function, so as to prevent the user from inputting a name that would cause a JavaScript or R error later on.

[alandipert]

@timelyportfolio OK, all changes are in 👍

[timelyportfolio]

thanks @alandipert !!!! merged!!!