docstr: add text string as header
  
  Args:
      text (str): text string written to pdf, with current
                  global indent level
      heading (str): heading elements of the text (h1 - h6)
      link (str): link idx created earlier for internal anchor
      style (bool): css style class
  Returns: HTML String
code: def add_header(text: str, heading: str, link=None, style=False):
      """
      add text string as header
  
      Args:
          text (str): text string written to pdf, with current
                      global indent level
          heading (str): heading elements of the text (h1 - h6)
          link (str): link idx created earlier for internal anchor
          style (bool): css style class
      Returns: HTML String
      """
      doc = Doc()
      with doc.tag(heading):
          if not (link is None):
              doc.attr(id="a%s" % link)
          if style:
              doc.attr(klass="section-title")
          doc.text(text)
      return doc.getvalue()
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import datetime
  from yattag import Doc, indent
label_desc: Use image node_id 17 for calling the CustomHtml obj's underlying member method code with example usage: obj.add_header(text, heading, link, style) and returns: doc