* Expand callouts functionality
Expand the callouts functionality to include a base set of types and allow for more customization.
* Grammar
* Include documentation for callouts in `README.md`
* .Site.IsServer was deprecated in Hugo v0.120.0, replaced by hugo.IsServer
* .Site.DisqusShortname was deprecated in Hugo v0.120.0, replaced by .Site.Config.Services.Disqus.Shortname
* Use .RenderString instead of markdownify (solves #104)
* The "tweet" shortcode requires two named parameters user and id.
* Replace .Site.BaseURL by absURL (solves #103)
Timo Montalto
Kyle Donnelly
Hyacinthe Cartiaux
athul