New MD syntax

The new MD flavor (aligned with github's Markdown) is added : V1. Opt-in for this release, will become default in the next major release.
V1 introduces strike and underline, and rehabilitates <b> and <i> tags.

  em: "an *italic* style"
  i: "an _italic_ style"
  strong: "a **bold** move"
  b: "a __bold__ move"
  u: "an ~underlined~ word"
  strike: "a ~~strike~~ out"

To opt-in for the new syntax:

let T = new MDText(texts, { MDFlavor: 1 });
// or for the singelton
T.setTexts(require('../texts/texts-en.yml'), { MDFlavor: 1, notFound: 'NA' });

MDText notFound property is deprecated - please switch to constructor or serTexts options.