Override methods which take an MdPlainText input to only provide a string

[Cloud-Awesome]

Started implementing during test refactoring, but most of the methods take this as an input.

e.g.

var table = new MdTable()
  .AddColumn("First Column")
  .AddColumn("Second Column")
  .AddColumn("Third Column");

instead of

var table = new MdTable()
  .AddColumn(new MdPlainText("First Column"))
  .AddColumn(new MdPlainText("Second Column"))
  .AddColumn(new MdPlainText("Third Column"));

Keep the original for backwards compatibility though

[Cloud-Awesome]

Required/Outstanding for following methods:

• [x] MdList.AddItem()
• [x] MdParagraph.Add()
• [x] MdQuote.AddLine()

During this, perhaps review:

• Naming of the above Add methods to make them more consistent
• Benefits of the MdPlainText class at all (as a public class at least)...