Add alt tags to images for a11y (#529)

- Add imageAlt to GridBlock
- Set alt equal to `user.caption` for mapped users
- Set alt equal to `post.author` for blog posts
- Update documentation

examples/basics/pages/en/index.js

@@ -177,7 +177,7 @@ const Showcase = props => {
     .map((user, i) => {
       return (
         <a href={user.infoLink} key={i}>
-          <img src={user.image} title={user.caption} />
+          <img src={user.image} alt={user.caption} title={user.caption} />
         </a>
       );
     });