Added a way to configure remark options.

[kakadiadarpan]

With remark, now the following options can be provided:

• gfm
• commonmark
• footnotes
• pedantic
• blocks

The config can be provided in gatsby-config.js as below:

  plugins: [
    {
      resolve: `gatsby-transformer-remark`,
      options: {
        gfm: false,
        commonmark: false,
        footnotes: false,
        pedantic: false,
        blocks: ["h2"],
     }
  },
// other plugins

Fixes #7992

[pieh]

LGTM

[kakadiadarpan]

Thanks for fixing the linting error @pieh! 😄