Monaco-editor: Implement folding provider for markdown

[purocean]

monaco-editor version: 0.13.1

      this.editor = window.monaco.editor.create(window.document.getElementById('editor'), {
        value: this.value,
        language: 'markdown',
        theme: 'vs-dark',
        fontSize: '20',
        folding: true
      })

not work :(

[rcjsuen]

Markdown support might not be complete in the Monaco Editor. You can try looking at this file for inspiration I suppose.