Detect file paths in metadata and add image sizes for a better layout

[bloodyowl]

https://github.com/image-size/image-size

Input:

---
title: foo
thumbnail: foo/bar.png
---

Output:

{
  "title": "foo",
  "thumbnail": "foo/bar.png",
  "__thumbnail_width": 800,
  "__thumbnail_height": 600
}