Install MDX integration (dev)dependencies

Dependencies:

- @babel/polyfill (1)
- @mdx-js/tag (2)
- camelcase (3)
- gatsby-image (4)
- gatsby-mdx (5)
- gatsby-plugin-sharp (6)
- gatsby-transformer-sharp (7)

Development Dependencies:

- rehype-slug (8)
- remark-breaks (9)
- remark-github (10)
- @mdx-js/mdx (11)

References:
  (1) https://www.npmjs.com/package/@babel/polyfill
  (2) https://www.npmjs.com/package/@mdx-js/tag
  (3) https://www.npmjs.com/package/camelcase
  (4) https://www.npmjs.com/package/gatsby-image
  (5) https://www.npmjs.com/package/gatsby-mdx
  (6) https://www.npmjs.com/package/gatsby-plugin-sharp
  (7) https://www.npmjs.com/package/gatsby-transformer-sharp
  (8) https://www.npmjs.com/package/rehype-slug
  (9) https://www.npmjs.com/package/remark-breaks
  (10) https://www.npmjs.com/package/remark-github
  (11) https://www.npmjs.com/package/@mdx-js/mdx

GH-129