Adding Frontmatter Reader: Support summary card (OG. Twitter)

[sspaeti]

See issues #1416 and specific description in my comment: #1416 (comment).

This works for my site dedp.online, but I'd appreciate some help to integrate it nicely. It's more hackable and hard-coded to these frontmatter I used:

struct FrontMatter {
    title: String,
    description: String,
    featured_image_url: String,
}

Would be nice to read them dynamically. Also, I'm not sure if I added it at the right points.

PS: I also tried to implement a preprocessor (see #1416 (comment)), but with this PR that one is not needed anymore. Also, I guess this should be integrated into mdBook, and can't be as Preprocessor only?