Add MdDocumentSet

[Cloud-Awesome]

To allow for the generation of multiple .md files in one .save call

e.g.

const string folderPath = "c:/output-documents/";

var documentSet = new MdDocumentSet(folderPath)
    .AddDocument(new MdDocument(...)
    .AddDocument(new MdDocument(...)
    .AddDocument(new MdDocument(...)
    .Save(); 

Good use cases for this are:

• Validate everything before generating anything
• Potentially expensive I/O calls can be saved to a single point, as opposed to every loop round a for each, etc.