extract tarball and convert md to html in tc-docs

gulpfile.js

let tarballPipe;
gulp.task('makeTarballPipe', async function() {
  let markdownStream = lazypipe()
  .pipe(markdown)
  .pipe(pug, {template: 'layout/layout.pug'});

  let tarStream = await tarball.getTarball();

  let tarPipe = tarStream
  .pipe(extract())
  .pipe(gulpif('*.md', markdownStream()));

  tarballPipe = tarPipe;
  return tarPipe;
});

gulp.task('webserver', ['makeTarballPipe'], () => {
  let s = site();
  s.add(tarballPipe);
  return s.pipe(virtual_webserver({debug: true}));
});