switch to commonmark

[McFoggy]

This PR is work in progress and is only submitted to show the attempt & progress on this subject & to allow discussions.

• introduce commonmark dependencies
• use commonmark for markdown rendering
• move RefLinks into a commonsmark-extension
• provide better handling/rendering for tasks, options:
  • use an AttributeProvider
  • extend an existing NodeRenderer
  • create an extensions
• ensure all mardown usage uses Markdown class
• remove markedj
• update existing tests, add additionals
• extend gitbucket to allow additional commonmark extensions
  • via Plugins mechanism
  • via configuration only and class availability in the classpath

Do not hesitate to help on this topic if you are interested in the subject.