code: function loadFile(filename, level, threshold) {
    var out = [];
    var file;
  
    out.push('<!-- BEGIN DOC-COMMENT H' + level + ' ' + filename + ' -->\n');
  
    try {
      file = fs.readFileSync(filename, {encoding: 'utf-8'});
      out.push(srcToMarkdown(file, level, threshold));
    } catch (e) {
      // I don't see how we can relay this error safely
    }
  
    out.push('<!-- END DOC-COMMENT -->');
  
    return out.join('');
  }
docs: Load file from disk and process to Markdown
  
  Visibility threshold can be:
  
  0: public only
  1: public and protected
  2: public, protected and private
  
  @param {String} filename  File name
  @param {Number} level     Header starting level (1-6)
  @param {Number} threshold Visibility threshold
  
  @return {String} Markdown text with placeholder envelope