Add server-side rendering

[jacklenox]

This PR adds a basic level of server-side rendering to Picard. It is dependent on #37 and the upgrading of Picard to be able to use v2 of WP-API. It has been rebased against that branch for this reason.

The premise is relatively simple:

• We use React's renderToStaticMarkup method to generate the markup needed.
• Instead of getting any data from the app, we feed in a dummy set of JSON data which contains placeholders.
• We wrap the markup in the PHP theming tags that are not present in our React code. E.g. <?php get_header(); ?> and <?php get_footer(); ?>.
• We replace our dummy placeholders with the correct PHP theming tags.
• We write the now complete markup into the relevant theme files.
• We celebrate. 🎉

What we end up with is automatically generated theme files whose markup is drawn from that found in our React components.

At present, I have placed all of this in a Webpack config file. This means that as part of a general Webpack build process that is set to watch our project, the files will be recompiled as we make updates.

(As a sidenote, I'm intending to switch the build process from Gulp to Webpack, for now though it just handles this automated file generation and doesn't actually do anything Webpacky.)

Thus, once this is all in place. We can pretty much forget about it and know that our server-side files will be updated as is necessary.

For now, and for the purposes of a proof of concept, only the index.php, page.php and single.php files are being generated. There is also scope to make this much DRYer, rather than just the same thing repeated over and over. However, for the purposes of showing how it works, I'm liking it exploded out for the time being.