Add Stirring theme as submodule

[hansliu]

Purpose

Stirring: a Pelican theme. The theme is used for hansliu.com blog.

Fully responsive, customizable for menu, lightbox for the image, cover for the article. Check out README for more information. Live demo website at here

The theme support Pelican 4.5.0

Approach

• Configuration everything as variable as possible.
• Add the latest submodule pelican-stirring as Stirring to the repo.

cc @getpelican, let me know if anything needs to be changed, thanks.

[justinmayer]

Thanks for the contribution, Hans.