Bump version: 1.2.3 → 2.0.0

README: expand the description of "YAML Front Matter"

change .dump to .proof

using the cascading property of CSS to get around with the .dump
problem (.dump was introduced to avoid ending in `,`)

add defaults

gitignore test/

Updated the description in the test

simplifies makefiles

Improve the doc

Added a link in the readme