Provide select and selectAll extension functions for RootElement

[twyatt]

It is idiomatic in D3 to perform selections on DOM and/or SVG elements, e.g.:

d3.select("tr")
    .selectAll("td")

In the above example, D3 users perform selections directly on UI/visual elements. It seems reasonable/convenient to provide an API that allows Krayon users to be able to perform selections directly from the RootElement, e.g.:

val root = RootElement()
root.selectAll(LineElement) // or: root.select(..)

This removes the ceremonious need to use asSelection, e.g.:

val root = RootElement()
root.asSelection()
    .selectAll(LineElement)