Load HTML into a graph

[baynezy]

Remove dependency on HtmlAgilityPack and replace with AngleSharp.

Update all code to still work with AngleSharp. Maintain the public interface for the Convertor class.

Completely refactor the project to approach the parsing of the HTML graph starting at the deepest node and working its way back up.

It's important that the node being processed is aware of its ancestry in the graph as this will influence how the node is processed.