enable mathjax, closes #39

[etiennebacher]

This PR is greatly inspired by John Coene's commit in cicerone. Note that MathJax won't be applied if the delay in setTimeout is lower than 400.

Here's an example without any other code in onafterchange:

library(shiny)

ui <- shinyUI(fluidPage(
  introjsUI(),
  withMathJax(),
  mainPanel(
    textInput("intro", "Enter an introduction"),
    actionButton("btn", "Press me")
  )
))

server <- shinyServer(function(input, output, session) {
  steps <- reactive(data.frame(
    element = c(NA, "#btn"),
    intro = c("$$y = x + 2$$", "$$y = x + 5$$")
  ))

  observeEvent(input$btn, {
    introjs(
      session,
      options = list(steps = steps()),
      events = list()
    )
  })

})

# Run the application
shinyApp(ui = ui, server = server)

This also works when there is other code in onafterchange. Replace events = list() by the following:

events = list(
    onafterchange = I("console.log('hello')")
 )

and run it in browser to check the console.

[carlganz]

This is great! Thanks so much. A few changes:

• [ ] Since MathJax won't always be enabled we need to add an if check so update the JavaScript like so if (window.MathJax) {setTimeout(function(){ MathJax.Hub.Queue(['Typeset', MathJax.Hub]); }, 400)}
• [ ] Let's remove the pasting JavaScript code in R and add the above JS code directly to the onafterchange callback in the rintro.js
• [ ] Update News file

Thanks again!

[etiennebacher]

I moved the code and added the condition. Concerning the NEWS.md, I added a new section but not sure if it's what you want.

Ah I just noticed that you already created a section for 0.3.0, but my branch wasn't up to date, I'll let you resolve the conflicts then (I don't think I can do something, right?) ;)