Refactor markdown rendering

components/md-renderer/mdx-components/a.js

@@ -0,0 +1,15 @@
+import styled from 'styled-components';
+
+const Link = styled.a`
+  font-weight: 600;
+`;
+
+const EnrichedLink = props => {
+  return (
+    <Link href={ props.href } target={ /^http(s)?:\/\//.test(props.href) ? '_blank' : '_self' }>
+      { props.children }
+    </Link>
+  );
+};
+
+export default EnrichedLink;