Add optional custom frontmatter meta description for SEO and Open Graph

[nielsenjared]

I appreciate the new SEO merge in #143. An additional feature I thought useful would be a custom meta description frontmatter field for SEO and Open Graph purposes. I'm relatively new to Gatsby, so I hope this PR is useful and not interfering with anything under the hood. Cheers!

Changes proposed:

• To src/pages/index.js and src/templates/blog-post.js add description to frontmatter query
• Add short-circuit evaluation for frontmatter.description in posts mapping (index.js) and SEO component (blog-post.js)
• Add description field to hi-folks/index.md to demonstrate use

[sidharthachatterjee]

Hey @nielsenjared

Thank you for opening this. We've moved our official starters over to our monorepo to help ease maintenance (gatsbyjs/gatsby#10310). Closing this since this repo is now a read-only mirror.

Please reopen this PR against the monorepo if you'd like.