Update header images

- Rename image.feature to header.image

_posts/2012-03-15-template-featured-image-external.md

@@ -1,6 +1,8 @@
 ---
 title: "Template: Featured Image (External URL)"
 date: 2012-03-15T15:15:12+00:00
+header:
+  image: https://farm5.staticflickr.com/4140/4939863887_84705982fd_b.jpg
 categories:
   - Template
   - Uncategorized

_posts/2012-03-15-template-featured-image-horizontal.md

@@ -1,6 +1,11 @@
 ---
 title: "Template: Featured Image (Horizontal)"
 date: 2012-03-15T15:15:12+00:00
+header:
+  image: unsplash-image-1.jpg
+  credit:
+    caption: Unsplash
+    url: https://unsplash.com
 categories:
   - Template
   - Uncategorized

_posts/2012-03-15-template-featured-image-text-readability.md

@@ -1,13 +1,14 @@
 ---
 title: "Template: Featured Image And Text Readability"
+header:
+  image: unsplash-image-4.jpg
+  credit:
+    caption: Unsplash
+    url: https://unsplash.com
 tags: 
   - sample post
   - readability
   - test
-image:
-  feature: sample-image-4.jpg
-  credit: WeGraphics
-  creditlink: http://wegraphics.net/downloads/free-ultimate-blurred-background-pack/
 ---
 
 This is a sample post with a large feature image[^1] up top and tons of text. Odio ad blue bottle vinyl, 90's narwhal commodo bitters pour-over nostrud. Ugh est hashtag in, fingerstache adipisicing laboris esse Pinterest shabby chic Portland. Shoreditch bicycle rights anim, flexitarian laboris put a bird on it vinyl cupidatat narwhal. Hashtag artisan skateboard, flannel Bushwick nesciunt salvia aute fixie do plaid post-ironic dolor McSweeney's. Cliche pour-over chambray nulla four loko skateboard sapiente hashtag.

_posts/2012-03-15-template-featured-image-vertical.md

@@ -1,6 +1,11 @@
 ---
 title: "Template: Featured Image (Vertical)"
 date: 2012-03-15T15:36:32+00:00
+header:
+  image: unsplash-image-6.jpg
+  credit:
+    caption: Unsplash
+    url: https://unsplash.com
 categories:
   - Template
   - Uncategorized