Calling an external file using an Rmd D3 chunk creates blank space or does nothing

[ronblum]

System details

r2d3 Version : 0.2.2
R Version    : 3.5.1

Steps to reproduce the problem

Use an R Markdown file to execute the sample bubbles D3 example from the gallery at https://github.com/rstudio/r2d3/tree/master/vignettes/gallery/bubbles

1. Create the flare.csv file, same as in the example.
2. Create a d3 chunk in an R Markdown file in RStudio that executes bubbles.js:

```{r setup, include=FALSE}
library(r2d3)
knitr::opts_chunk$set(echo=FALSE, comment=NA)
```

```{d3 data = read.csv("flare.csv"), d3_version = 4, script = "bubbles.js"}

```

3. Run all the chunks.
4. Knit the file.

Describe the problem in detail

The following error occurs:

1. When running the chunks, nothing happens.
2. When knitting the file, a bunch of blank space is displayed. Resizing the display window or including something after the d3 chunk, such as an r chunk with plot(cars), will make this clear.

Describe the behavior you expected

The D3 portion should be displayed correctly. Compare this to the independent D3 JavaScript file:

1. Save a copy of the bubbles.js file in the same directory as the flares.csv file.
2. In RStudio: File -> Open File -> bubbles.js
3. Preview

The D3 file is rendered correctly, including the bubbles image.

Note: If calling a file is not supported, then this should error out instead of displaying blank region or nothing.