add support for subtitle and post thumbnail image

[HRHDaniel]

@artemsheludko I found I wanted a bit more control over the index page listing, specifically:

• Added an option for a different image customized for the index page: thumb: post-thumbnail.png
• Added an option for a subtitle, rather than relying on the first 15 words of the post: subtitle: "Description of the post for the index page"

Both are optional and default back to the existing behavior, so completely backwards compatible.

Thanks for the great theme!