Add mirage get post route

We configure Mirage to return a post retrieved by ID. This allows the post detail screen to be displayed.

Outer Red:
✘ Element .js-post-detail-title has text "Test Post"
✘ Element .js-post-detail-body has text "This post is a test!"

The title is not shown because the data from our create post form isn't actually submitted.

mirage/config.js

@@ -1,3 +1,4 @@
 export default function() {
   this.post('/posts');
+  this.get('/posts/:id');
 }