Refactor seo.html and support OpenGraph image overrides (#1321)

- Refactor seo.html include and add `page.header.og_image` override
- DRY up handling of page images (`page.header.image`, `page.header.overlay_image`, `page.header.teaser`, and `site.og_image`).
- Allow page images to be set with `header.og_image` page variable in YAML Front Matter as per feature request in #1316.
- Add sample posts for testing `og_image` override
- Set author bio as site OpenGraph default image
- Document `page.header.og_image` OpenGraph override
- Update CHANGELOG and history

Close #1316

test/_posts/2010-08-05-post-header-image-og-override.md

@@ -0,0 +1,22 @@
+---
+title: "Post: Header Image with OpenGraph Override"
+header:
+  image: /assets/images/page-header-image.png
+  og_image: /assets/images/page-header-og-image.png
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - edge case
+  - image
+  - layout
+last_modified_at: 2017-10-26T15:12:19-04:00
+---
+
+This post has a header image with an OpenGraph override.
+
+```yaml
+header:
+  image: /assets/images/page-header-image.png
+  og_image: /assets/images/page-header-og-image.png
+```

test/_posts/2010-08-05-post-header-overlay-image-og-override.md

@@ -0,0 +1,26 @@
+---
+title: "Post: Overlay Image with OpenGraph Override"
+header:
+  overlay_image: /assets/images/unsplash-image-1.jpg
+  og_image: /assets/images/page-header-og-image.png
+  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
+  cta_url: "https://unsplash.com"
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - edge case
+  - image
+  - layout
+last_modified_at: 2017-10-26T15:12:19-04:00
+---
+
+This post has a header image with an OpenGraph override.
+
+```yaml
+header:
+  overlay_image: /assets/images/unsplash-image-1.jpg
+  og_image: /assets/images/page-header-og-image.png
+  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
+  cta_url: "https://unsplash.com"
+```

test/_posts/2010-08-05-post-teaser-image-og-override.md

@@ -0,0 +1,22 @@
+---
+title: "Post: Teaser Image with OpenGraph Override"
+header:
+  teaser: /assets/images/page-header-teaser.png
+  og_image: /assets/images/page-header-og-image.png
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - edge case
+  - image
+  - layout
+last_modified_at: 2017-10-26T15:12:19-04:00
+---
+
+This post has a teaser image with an OpenGraph override.
+
+```yaml
+header:
+  teaser: /assets/images/page-header-teaser.png
+  og_image: /assets/images/page-header-og-image.png
+```