Inline output in R Markdown documents

This PR adds an `inline` argument to four output functions: textOutput(), imageOutput(), plotOutput(), and htmlOutput(), to make it possible to write output to `<span>` instead of `<div>`.

Here is a test document: https://yihui.shinyapps.io/shiny-inline/

    ---
    title: "Inline Output"
    runtime: shiny
    output: html_document
    ---
    
    You can embed Shiny inputs and outputs in your document. Outputs are
    automatically updated whenever inputs change. This demonstrates how a standard R
    plot can be made interactive by wrapping it in the Shiny `renderPlot()`
    function. The `selectInput()` and `sliderInput()` functions create the input
    widgets used to drive the plot.
    
    ```{r tidy=FALSE}
    sidebarLayout(
      sidebarPanel(
        selectizeInput('main', 'Main title', LETTERS),
        sliderInput('size', 'Point size', min = 0.2, max = 5, value = 1)
      ),
      mainPanel(
        renderPlot(plot(cars, main = input$main, cex = input$size, pch = 19),
                   width = 600, height = 400)
      )
    )
    sunsplots_line = function() {
      par(mar = rep(0, 4))
      plot(sunspots, axes=FALSE, ann=FALSE, asp=input$asp)
    }
    ```
    
    The main title is `r renderText(input$main)` and the point size is `r renderText(input$size)`.
    
    Besides `renderText()`, you can also embed some other output elements inline.
    
    For example, here is a spark line `r renderPlot({sunsplots_line()}, width=300, height=40)`
    that shows you the time series `sunspots`.
    
    ```{r}
    sliderInput('asp', 'Aspect ratio', .02, .3, .2)
    ```