demo htmlwidgets::JS() with tooltipOptions

dev/dev.Rmd

 * it works!
 * Try adding other options
 
+### Writing JavaScript in R
 
+The [tooltips](https://frappe.io/charts/docs/basic/annotations#tooltips)
+can be formatted using the `tooltipOptions` property:
+
+```
+tooltipOptions: {
+    formatTooltipX: d => (d + '').toUpperCase(),
+    formatTooltipY: d => d + ' pts',
+}
+```
+
+To write this in R (add to `widget_demo.R`)
+
+```r
+tooltipOptions = list(
+  formatTooltipX = htmlwidgets::JS("d => 'Year: ' + d"),
+  formatTooltipY = htmlwidgets::JS("d => d + ' babies'")
+)
+```

dev/widget_demo.Rmd

   lineOptions = list(regionFill = TRUE),
   axisOptions = list(xIsSeries = TRUE),
   colors = c("#466683", "#44bc96"),
-  width = "100%"
+  width = "100%",
+  tooltipOptions = list(
+    formatTooltipX = htmlwidgets::JS("d => 'Year: ' + d"),
+    formatTooltipY = htmlwidgets::JS("d => d + ' babies'")
+  )
 )
 ```