Update social link in Bio component

Start using social object from gastby-config

Author: lukewhitehouse

gatsby-config.js

@@ -5,7 +5,7 @@ module.exports = {
     description: `A starter blog with added site search.`,
     siteUrl: `https://gatsby-starter-blog-with-lunr-demo.netlify.com/`,
     social: {
-      twitter: `_lukewh`,
+      twitter: `https://twitter.com/_lukewh`,
     },
   },
   plugins: [

src/components/bio.js

@@ -36,7 +36,7 @@ function Bio() {
                 }}
               />
               <p>
-                This project is a fork from <a href="https://github.com/gatsbyjs/gatsby-starter-blog">Gatsby's starter blog</a>, with added Site Search functionality thanks to <a href="https://lunrjs.com/">Lunr.js</a>. It is maintained by <a href="https://twitter.com/_lukewh">{author}</a>, who wrote a tutorial on <a href="https://assortment.io/posts/gatsby-site-search-lunr-js">building this from scratch</a>. 
+                This project is a fork from <a href="https://github.com/gatsbyjs/gatsby-starter-blog">Gatsby's starter blog</a>, with added Site Search functionality thanks to <a href="https://lunrjs.com/">Lunr.js</a>. It is maintained by <a href={social.twitter}>{author}</a>, who wrote a tutorial on <a href="https://assortment.io/posts/gatsby-site-search-lunr-js">building this from scratch</a>. 
               </p>
             </div>
             <p style={{ marginBottom: rhythm(2.5) }}><Link to="/search">Search all posts</Link></p>