code: function transformChunk(chunk, encoding, done) {
      if (chunk.isNull()) return done();
      if (chunk.isStream()) return this.emit('error', new PluginError(logFlag, 'Streaming not supported'));
      if (
        !(chunk.isMarkdown = options.regexpMarkdown.test(chunk.relative)) &&
        !(chunk.isTemplate = options.regexpTemplate.test(chunk.relative)) &&
        !(chunk.isHtml = options.regexpHtml.test(chunk.relative))
      ) {
        return options.passThrough ? done(null, chunk) : done();
      }
      transformChunkData(chunk);
      if (chunk.data.draft && process.env.NODE_ENV !== 'development') return done();
      if (buffer.hasOwnProperty(chunk.relative)) {
        logDuplicate(chunk);
        return done();
      }
      transformChunkContents.call(this, chunk);
      buffer[chunk.relative] = chunk;
      done(null, chunk);
    }
docs: transforms a stream chunk
  @param  {object}   chunk    chunk object
  @param  {string}   encoding file encoding
  @param  {Function} done     callback
  @return {undefined}