Next.js: add purge css in tailwind css example

[itsjzt]

Feature request

Is your feature request related to a problem? Please describe.

Tailwind is a heavy framework and using it without purge CSS can create a very heavy CSS file.

Describe the solution you'd like

tailwind CSS example should also use purge css plugin of postCSS

[shaunchurch]

I've just enabled this today in a project I'm working on, so should be able to send in a PR later.

[taylorbryant]

I put up https://github.com/zeit/next.js/pull/9145 to address this issue.