Gatsby Plugin: Add SourceInstanceName to MarkdownRemark

# With Yarn
$ > yarn add -D gatsby-source-instance-name-for-remark

# With NPM
$ > npm install --save-dev gatsby-source-instance-name-for-remark

// gatsby-config.js

module.exports = {
  siteMetadata: {
    title: "Gatsby Default Starter"
  },
  plugins: [
    {
      resolve: "gatsby-source-filesystem",
      options: {
        path: `${__dirname}/pages`,
        name: "pages"
      }
    },
    {
      resolve: "gatsby-source-filesystem",
      options: {
        path: `${__dirname}/products`,
        name: "products"
      }
    },
    "gatsby-transformer-remark",
    "gatsby-source-instance-name-for-remark"
  ]
};