highcharts: add exporting feature

Related to issue #171. 

To enable the exporting features, it requires additional dependencies, hence this pull request. I have set it as `enabled = F` as default. For an example on how to use, see below:

```
a <- hPlot(Pulse ~ Height, data = MASS::survey, type = "bubble", title = "Zoom demo", subtitle = "bubble chart", size = "Age", group = "Exer")
a$chart(zoomType = "xy")
a$exporting(enabled = T)
a
```

Successfully tested with the highcharts examples in the package.