Embed interactive .html plot in documentation using Documenter.jl

[cyrieln]

I found a solution:

```@example
using PlotlyJS

plt = plot(scatter(x=1:10, y=1:10))

savefig(plt, "figure.html")
```

```@raw html
<iframe src="figure.html" style="height:500px;width:100%;"></iframe>
```