from_spec: Take a JSON Vega-Lite Spec and render as an htmlwidget
Description
Vega-Lite is - at the core - a JSON "Grammar of Graphics" specification
for how to build a data- & stats-based visualization. While Vega & D3 are
the main targets, the use of Vega-Lite does not have to be restricted to just
D3. For now, this function takes in a JSON spec (full text or URL) and
renders it as an htmlwidget. Data should either be embedded or use a
an absolute URL reference.