Allow optional addition of social links to sidebar

[alanorth]

It would be pretty compatible with blog conventions to have your Twitter, Google+, Facebook, etc links on the sidebar. It should be optional and ideally use the existing config options where possible (like params.social.twitter, which is already used by Twitter cards).

This could perhaps be enumerated like a menu, or like the social sharing links, and we could show Font Awesome icons for each link.

[Jieiku]

Here is a theme that uses SVG Paths for the Social sharing link images, so no Font Awesome is even necessary: https://themes.gohugo.io/theme/hugo-alageek-theme/ he does not have facebook, pinterest, or instagram, but he does have pretty much all the other ones. Maybe not too difficult to create SVG paths for any remaining social networks.

[alanorth]

I'm not very worried about including Font Awesome. We don't even load all of it (see: source/scss/font-awesome.scss), and I only import about half of Bootstrap as well. If anything I would think about moving to Font Awesome 5 and then adding some sort of configuration mentioned in the issue to allow putting social icons in the sidebar.