Allow for a chart to be built inside of existing elements

I think it will be useful to be able to build charts inside of existing elements. I added logic to `scaffoldChart` that does the following.

if the container is a `svg`, make a chart in that `svg`
if the container is a `g`, make a chart in that `g`
else the container is something else, append an `svg` and make a chart in that `svg`

As a result I renamed `this.svg` to `this.container`