Add a basic POCO - Markdown converter

[Cloud-Awesome]

1. Create a document passing a POCO in the constructor

public MdDocument(string fileName, IDocumentMe pocoClass, IFileSystem fileSystem = null)

2. Create a document passing in a list of POCOs in the constructor

public MdDocument(string fileName, IEnumerable<IDocumentMe> pocoClasses, 
            IFileSystem fileSystem = null)

3. Create a new IDocumentPart type to enable it to be embedded into a wider document