Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

tpl/urls/urls.go

@@ -29,7 +29,7 @@ import (
 func New(deps *deps.Deps) *Namespace {
 	return &Namespace{
 		deps:      deps,
-		multihost: deps.Cfg.GetBool("multihost"),
+		multihost: deps.Conf.IsMultihost(),
 	}
 }