Add property in links param to modify jumbotron button variants

The current frontmatter param for links looks like:

links: [[href: "about/", text: "About"], [href: "news/", text: "News"]]

It would be nice to have a property named class. e.g.

links: [[href: "about/", text: "About"], [href: "news/", text: "News", class: "btn btn-secondary" ]]

Currently, we only have jumbotron_btn_class, but that changes all buttons. What I am proposing would give us control over individual buttons.

I think the jubmotron_btn_class could serve as a default. The class property should be able to override it.