Support images in the front matter

[natikgadzhi]

If the front matter key (database field) is an attachment, try processing it, and return a path to the attachment on the web server.

The initial implementation will be affected by #25. If we preprocess and copy all images, then the front-matter key will need to return the path to the image on the web server that's generated by the user's blogging engine, this will need to be a setting.

The initial implementation may be limited to just images — it's the most common type of an embedded attachment in the front-matter anyway.