Cover image for single post

[oswaldoacauan]

[stevelacey]

I've seen other themes use a tag to indicate that the first image of the post should be grabbed and used as a cover image, and you could always do the same by sticking class on the image. A bit hacky, but gets it done, and this would be a real nice feature to add.

I'd PR an implementation, but I can't find a drop in Medium-style article image CSS/JS implementation anywhere.