Adding Emoji Support to MDX files

The remark plugin remark-emoji will transform short codes like :rocket: into their unicode equivalent (🚀). It's not a transformer plugin, it's remark plugin, so config is a little different to normal:

$ npm install --save remark-emoji

  {
    resolve: "gatsby-plugin-mdx",
    options: {
      remarkPlugins: [
        require('remark-emoji')
      ]
    }
  }