[1.0] Add support for 'internal' plugins

[KyleAMathews]

No description provided.

[KyleAMathews]

Move the automatic page creation for react components to an internal plugin that uses the standard createPages plugin. I eventually want to move this code out into a standalone module anyways so this is a good first step. We're blocked on doing that until Gatsby has theme support where we can create a default core theme with more-or-less required plugins that then all other themes can build off of.

Internal plugins are a nice modular way for building modular required functionality into Gatsby without hard-coding it in awkward ways around the Gatsby package's code.

[KyleAMathews]

Deploy preview failed.

Built with commit f040052

https://app.netlify.com/sites/using-postcss-sass/deploys/5920465ea700c44283514728

[KyleAMathews]

Deploy preview failed.

Built with commit f040052

https://app.netlify.com/sites/gatsbyjs/deploys/5920465ea700c44283514720

[KyleAMathews]

Deploy preview failed.

Built with commit f040052

https://app.netlify.com/sites/gatsbygram/deploys/5920465ea700c44283514722

[gatsbybot]

Deploy preview failed.

Built with commit f040052

https://app.netlify.com/sites/using-drupal/deploys/5920465ea700c44283514724

[gatsbybot]

Deploy preview ready!

Built with commit 0dbd702

https://deploy-preview-1010--using-drupal.netlify.com

[gatsbybot]

Deploy preview ready!

Built with commit 0dbd702

https://deploy-preview-1010--gatsbygram.netlify.com

[gatsbybot]

Deploy preview ready!

Built with commit 0dbd702

https://deploy-preview-1010--gatsbyjs.netlify.com