v2.1.80-rc

• Resolve constructor ambiguities
• Add "MdAdmonition" block type
• Add IReadOnlyCollection implementation and Insert() to MdCompositeSpan
• Add IReadOnlyList implementation and Insert() to MdTableRow
• Add IReadOnlyCollection implementation to MdTable
• Implement IReadOnlyCollection and add Insert() to MdContainerBlock and MdList
• Add MdTable.Insert()
• Add MdDocumentSet
• Add convenience constructors to MdEmphasisSpan and MdStrongEmphasisSpan to allow passing in multiple spans.
• Add SyntaxVisualizer public
• Fix more construtor ambiguities in container blocks
• Add DeepEquals() method to all block and span classes
• Add MdHeading.Anchor property to make linking to sections of a document easier
• Add MdEmptyBlock
• Fix typos in comments and inline documentation
• Add MdParagraph.Add() method